Celtic completed their first signing of the summer transfer window earlier this week when they announced that Kieran Tierney has re-joined the Glasgow giants.

The experienced defender will officially link up with his new teammates when his contract with Arsenal expires at the end of the month, returning to Scotland as a free agent.

Tierney came up through the academy system at Parkhead and won five Scottish Premiership titles during his time in the first-team, a haul that he will now be looking to add to.

The 28-year-old full-back amassed 37 assists in 170 appearances for the Scottish giants in all competitions, and he will be aiming to provide that sort of creativity down the left flank for Brendan Rodgers.

Tierney is set to come in as a direct replacement for Greg Taylor, whose Celtic contract expires this summer, but the Hoops have also lost their second-choice left-back, as Jeffrey Schlupp's loan has come to an end.

Why Celtic need to replace Jeffrey Schlupp

Celtic will play in four competitions next season: the Champions League, the Premiership, the League Cup, and the SFA Cup. Because of this, having depth across the park in every position is vital for Rodgers.

It is not enough to just have one player in each position, and that is why the Bhoys have to sign a back-up left-back to replace Schlupp, particularly when you take Tierney's injury history into account.

The Arsenal left-back has missed 45 matches through injury since the start of the 2023/24 campaign, which means that there is a concern that he will not be available on a consistent basis for Rodgers next term.

This emphasises the importance of signing a replacement for Schlupp because the Hoops will need a left-back who is good enough to step in and start when Tierney misses games through injury over the course of the season.

As you can see in the table above, Schlupp was needed to start eight matches in the Premiership and the Champions League, despite only joining in January, which shows how important it is to bring in a quality back-up option in that position.

Celtic, therefore, need a left-back who provided the quality, particularly defensively, that Schlupp provided during his five-month loan spell at Parkhead.

The Scottish giants are in the market for another option in that position and their latest reported target could be an even better signing than the Palace loanee was.

Celtic progressing in deal for Scottish defender

According to a recent claim by journalist Graeme Bailey, Celtic are progressing in their pursuit of a deal to sign Josh Doig from Sassuolo in the summer transfer window.

The reporter was speaking about Harry Toffolo's situation, after it was confirmed that he is leaving Nottingham Forest on a free transfer, and all but ruled out the Hoops in the race for his signature, due to their interest in Doig.

Bailey told EFL Analysis: “He’s an interesting one. I did hear a little bit of a squeak for Celtic, but I don’t think there’s anything in that. I do know Celtic are looking at Josh Doig and I think that one’s progressing, to be fair."

This suggests that the Hoops are moving forward in a deal to bring the Scottish defender back to Scotland, in what would presumably be a permanent move, given that he was just a key part of Sassuolo's title-winning season in the Serie B.

If Celtic can finalise a transfer to sign Doig from the newly-promoted Serie A outfit before the end of the summer transfer window, then they could have an even better signing than Schlupp.

Why Josh Doig would be a better signing than Jeffrey Schlupp

Perhaps the biggest reason why he would be a better signing than the Palace loanee was would be the permanent transfer aspect of it, as it would be a long-term solution to that position.

Schlupp was only a short-term stop-gap until the summer, whereas Doig is 23 and could be Celtic's back-up, or first-choice, left-back for many more years to come.

This means that Celtic could get far more long-term value out of this signing, should they sign him permanently, if he adapts well and performs on the pitch as hoped.

Doig, who former boss Gabriel Coffi claimed has "incredible potential", also put in good enough performances on the pitch in the Serie B this season to suggest that he could be a quality addition to the squad when called upon as a back-up to Tierney.

As you can see in the table above, the 23-year-old star could be well-suited to playing in a ball-dominant Celtic team, particularly in a similar role to the one that Taylor played this term, inverting into a midfield position in possession.

These statistics show that Doig is incredibly comfortable and progressive in possession, excelling as a passer and a carrier of the ball, which suggests that he would thrive under the unique demands that Rodgers places on his left-back.

Therefore, the Sassuolo star, who started 25 Serie B matches, could be a perfect fit for what Celtic are looking for in that position, which is another reason why he would be a terrific signing for the club.

Overall, he would be an even better signing than Schlupp due to his age, with plenty of room left to improve, his long-term potential as a signing due to it being on a permanent deal, and because of the quality that he can produce on the pitch.