When news broke in March that Kieran Tierney had suffered a knee injury that would rule him out of Scotland’s delayed World Cup play-off semi-final against Ukraine, now taking place at Hampden on Wednesday, there were probably a few wails of dismay.

The exasperation stemmed not only from Tierney being one of Scotland’s best players but because he is a vital cog in Steve Clarke’s well-oiled machine. Since his appointment three years ago, the manager has settled into a general formation, a game plan within that formation, and used regular personnel.

A back three or five, which defends narrowly and compactly, have been increasingly proactive in their pressing and more patient in possession over the last year.

If Tierney was available, you could confidently predict eight of the starting XI against Ukraine — all except the middle and right-sided centre-backs, and either Lyndon Dykes or Ryan Christie in attack, depending on whether Clarke opts for a 3-5-2 or the 3-4-2-1 which has proved so effective this season. Nathan Patterson has been noted as a doubt for the Ukraine game due to a lack of match fitness, in which case one of Stephen O’Donnell or Anthony Ralston will feature instead, but this assumes he is given the eventual nod.

Arsenal’s former Celtic star was an automatic pick as the left-sided centre-back, from where he was not only an effective defender but one of their main attacking assets.

A key tactic has been Tierney’s pace and delivery when overlapping Andy Robertson as the left wing-back, who then drops to cover Tierney’s surge forward.

There were two great examples of this during the 4-0 win over the Faroe Islands last March, when Tierney assisted John McGinn twice in succession.

That, on top of Tierney’s solidity, is difficult to replace. Yet that is the task now facing Clarke ahead of one of Scotland’s biggest games this century.

Here are the potential contingency plans, listed in order of descending unlikeliness with the final possibility the zaniest of all — switching to a back four.

The simplest solution to losing a left-footed centre-back is to replace him with a left-footed centre-back.

There are two of those in the squad for a set of June fixtures that will hopefully include a play-off final away to Wales next Sunday: Liam Cooper of Leeds United and Nottingham Forest’s Scott McKenna (top photo). Both have experience in a back three and are coming off positive individual seasons.

Although he has not always excelled with Scotland, Cooper was outstanding in the 2-0 win over Denmark which confirmed their top-seed status for the play-offs. He suffered an injury-ravaged campaign with Leeds as they flirted hard with relegation from the Premier League, but when he did play he was an assured presence in an often turbulent defence. Of the two options, he is also marginally the faster player and possibly better in possession, which would partly compensate for Tierney’s range of passing and ability to play out under pressure.

Aberdeen academy product McKenna has been one of the English Championship’s steadiest centre-backs during Forest’s climb from last place in September to almost earning automatic promotion under new manager Steve Cooper. He has been defensively secure but also improved his ability on the ball — arguably his biggest weakness at Aberdeen — although his lack of pace and front-foot style of defending can leave him exposed.

McKenna is likely to feature for Forest in the Championship play-off final against Huddersfield Town at Wembley, which is only three days before the Ukraine game and could go to extra time — something that will surely factor into Clarke’s thinking.

Both players have familiarity with the role and are dogged defenders but lack the athleticism and attacking contribution Tierney offers.

The graphic below shows Tierney’s touches as the left-sided centre-back during the 1-1 draw with Poland in March. That intention to get forward and contribute in attack through overlapping is clear through both the number of touches in the opposition half, and how wide some of those touches are.

The next graphic is McKenna’s touch map in Forest’s 2-1 loss to Sheffield United 10 days ago in the play-off semi-final second leg, from which they progressed after a penalty shootout. He was also playing as the left-sided centre-back.

Naturally, a degree of caution is needed because Cooper’s Forest do not play identically to Clarke’s Scotland, but it also illustrates that McKenna is more of an old-school centre-half.

So if Clarke wanted to try to replicate Tierney’s attacking contributions, he could try a more experimental alternative…

The first rogue option to maintain that attacking threat could be starting Aaron Hickey as the left-sided centre-back.

Hickey is a versatile 19-year-old who has starred for Bologna in Serie A this season and has already been linked with a move to join Tierney at Arsenal. He was capable of playing right-back or left-back for former club Hearts, and he has switched between left and right wing-back in his two years at Bologna. He made his international debut against Poland and then played against Austria in the March friendlies, each time off the bench as the left wing-back.

Where he has not played, however, is left-sided centre-back.

Yet Hickey’s versatility is down to three key factors; how two-footed he is, his competence in possession and his defensive security. He reliably puts in good performances regardless of where he is deployed on the pitch.

That dependability is complemented by his physical growth since leaving Scottish football after the 2019-20 season. Now standing 6ft tall and having bulked up, he is potentially better suited to the more arduous responsibilities of a back-three centre-half.

With his speed and ability in possession, Hickey would provide recovery pace to sweep up long balls in behind and offer Clarke a more technically capable option in playing out from the back.

He could potentially replicate the Tierney/Robertson overlap, although his final ball has room for improvement.

For Bologna this season, he averaged 0.3 chances created per 90 minutes and 0.11 expected assists (xA) per 90 according to Instat, compared to Tierney’s 0.4 chances created and 0.17 xA per 90 for club and country.

Ultimately, though, starting Hickey in a role in which he is unfamiliar in a game of such importance would be a major risk — even if it would be the ideal occasion for him to pull off something like this…

An alternative to having Hickey as the left-sided centre-back would be switching him with Robertson, and playing Hickey (or Greg Taylor) as the left wing-back.

Robertson’s ability on the ball is well-established and, with his pace and excellent delivery, he could potentially emulate Tierney’s tactic of overlapping beyond the wing-back. His defensive diligence and ability in the air are arguably underrated too, with Instat noting he has won 62 per cent of his aerial duels for club and country in 2021-22.

Like Hickey, the Liverpool man has the recovery pace to protect the space behind a back three too.

Robertson, however, would be unfamiliar with the role — Liverpool manager Jurgen Klopp rigidly adheres to a back four, while Robertson has exclusively featured as the wing-back in Clarke’s back five.

Then there is the additional problem of moving one of your best players out of position to compensate for the loss of another of your best players, potentially undermining the coherence of Clarke’s system even further.

If this were to happen it would presumably be Robertson at left-back, with Clarke simply opting for his two first-choice centre-backs.

This is possibly the least likely option for Clarke.

Firstly, having played almost exclusively in a back three for three years, it would be a risk to switch it up now. Even if they train rigorously in a back four during their pre-Ukraine training sessions, after dozens of games in a back three, the players have become intuitively accustomed to their in-game roles and responsibilities, which can be a different challenge to training exercises.

Secondly, there are no natural wingers in this squad.

They could instead play with two No 10s in a 4-3-2-1, which might work, in theory, with Robertson and probable right-back Nathan Patterson’s inclinations to overlap and provide the missing width. Another midfielder, presumably Ryan Jack, could also help Scotland exert control over the middle of the pitch. That is a faint possibility, but Clarke’s squad selection and commitment to a back three suggest it is almost certainly not on the cards.

So while the Cooper/McKenna route is the most likely one the manager will go down, there are a couple of alternative strategies to consider to preserve the attacking and creative threat down that left flank too.

 (Top photo: Craig Williamson/SNS Group via Getty Images)