If there was one big takeaway from Celtic's derby defeat on Sunday, it was just how important Yang Hyun-jun is to the fortunes of Martin O'Neill's side.

In the absence of the South Korean, the Hoops boss again handed Sebastian Tounekti a chance to impress, the Tunisian previously producing a mixed bag performance away in Perth in midweek.

Surprise surprise, the 24-year-old did not take the opportunity afforded to him, subsequently hooked at half-time following another lifeless display down that left flank.

Of course, it would be wrong to suggest that he's the only issue facing the champions right now, but Tounekti's long-standing woes have brought the departures of previous Celtic assets into even sharper focus.

Why Celtic have reached the end of the road with Tounekti

It may appear too soon to be writing off any player at Parkhead, considering the season is still in its relative infancy, yet Tounekti's woes stretch back to what was a disappointing debut campaign last time out.

Signed for a reported fee of around £5.2m, expectations were high following his arrival from Hammarby, although Celtic just haven't seen a return on their investment so far.

Perhaps this is merely what the club should have expected considering the fleet-footed winger actually only scored three and assisted five for his former employers, in 27 outings, while previously registering just four goals and five assists in 47 games for FK Haugesund.

As for his time in Glasgow so far, Tounekti has registered only six goals and four assists in 55 games, a desperately poor return for a forward in a side that has been so dominant domestically.

His only goal involvements this season came in the League Cup against Dundee United, having still only contributed two goals and two assists in the Premiership in total across the last 12 months or so.

The return of Yang, and Jota, simply can't come soon enough, it must be said, with O'Neill likely to have to resort to relocating key man Camilo Duran on the left in the short term.

It doesn't look likely that Tounekti will be able to recover from this slump, desperately short on confidence and end product right now, as the weekend defeat showcased.

There must be those at Parkhead rueing the loss of what would have been a clear upgrade.

Celtic already let a Tounekti upgrade leave for free

From Luis Palma, to Michel Ange Balikwisha, to Tounekti, that left flank has been an issue for a number of years now in Glasgow, not least with Jota's return having not gone smoothly over the last 18 months or so.

James Forrest, a man comfortable on either flank, remains a trusty, reliable servant to the club, the veteran winger one of the few academy success stories of modern times.

Callum McGregor and Kieran Tierney are in that boat too, with Colby Donovan hoping to follow, but time and again the Hoops have seen their own products go on to shine elsewhere, be it Ben Gannon-Doak or even Calvin Miller.

Miller's case is a particularly intriguing one, considering his rise in recent years, the 28-year-old having emerged as one of the Premiership's standout widemen, six years on from leaving Celtic Park.

Signed by the club as a youngster back in 2010, the Glaswegian rose through the ranks as a forward or winger, albeit with Brendan Rodgers and co repurposing him as a left-back at first-team level.

Miller even made his debut in that role against Partick Thistle in December 2016, albeit while going on to make just four more appearances in total over the next few years, even while signing a contract extension in 2018.

After muted spells at Dundee United and Ayr United on loan, a significant injury ultimately curtailed his Celtic career, the versatile talent departing at the end of his deal on a free in May 2020.

Having initially travailed the lower leagues of English football, Miller's stock has soared since returning to Scotland in 2022, his move to Falkirk a year later really sparking a change in fortunes.

In 2024/25, for instance, he was named the Championship's Player of the Year, maintaining that momentum last term too to help Falkirk to a remarkable top-six finish.

While Tounekti toiled with his four goals and assists in the top-flight, Miller, now operating primarily as an out-and-out winger or number ten, scored and assisted 16 goals in the Premiership, creating 18 'big chances' and averaging 1.6 key passes per game.

Tounekti, meanwhile, created just three 'big chances' and averaged only 1.4 key passes per appearance.

Subsequently snapped up by Hearts, Miller has already scored four times this season in all competitions too, helping the Jambos to see off Rapid Vienna in Conference League qualifying.

In the view of Scott Brown, he deserved to be in the conversation for a Scotland call-up following his form at Tynecastle, albeit with teammate and former Rangers man Josh McPake among those to get the nod instead in Sebastien Pocognoli's new-look squad.

From a Celtic perspective, it will remain a frustration that 'one of their own' is now shining elsewhere, with Miller up to 24 goal involvements since the start of last season in all competitions, well ahead of Tounekti's return of 11.

Perhaps the Scot needed to leave to get to where he is now, but in any case, he's comfortably clear of Tounekti, a fact that can't be disputed.