The Hoops have lost three of their four matches, drawing the other one, as the Australian has been unable to muster up a win in the competition.
He is not the first manager of the Glasgow giants to struggle in Europe, though, and one man who failed to achieve success on the continent was Ronny Delia.
The Norweigan manager was knocked out of the Champions League in the qualifying rounds in both of his seasons at Celtic Park, whilst also failing to make it to the last 16 of the Europa League in those campaigns.
He struggled badly in European competitions and was also no stranger to a big blunder in the transfer market during his time in Glasgow.
Former Hoops full-back Tosh McKinlay showered the centre-back with praise at the time of the move, saying:
“Simunovic will be top drawer. He’s different class. This is a guy who has already played over 50 times for Dinamo Zagreb, has captained Croatia’s Under-21s and has great pedigree.
“I have seen a lot of footage of him, he’s a fantastic footballer. Simunovic is Rolls Royce material. He will make a big impact in the Scottish game.”
TheCelticBlog previously described him as ‘error prone’ and claimed that he had made ‘calamitous’ mistakes, whilst former Hoops shot-stopper Pat Bonner once stated that the club had seen big errors from the defender “so many times“.
Celtic eventually decided to part ways with the centre-back at the end of his contract in the summer of 2020, allowing him to leave on a free transfer.
This meant that the Hoops did not recoup a single penny of the significant £5.5m they paid for him in 2015 and were not rewarded with a great deal on pitch, as he struggled with injuries and became too prone to making mistakes when he was available to feature.
Therefore, Delia dropped a clanger by splashing out to sign the flop in the summer of 2015 and it is not a transfer that should be looked back on as a success.