With fans protesting against the club's board at the start of the game and later ironically cheering Schmeichel making a save, the atmosphere at Parkhead was far from harmonious.

"It's bitterly disappointing," former Celtic captain and manager Neil Lennon said of the jeers directed at Schmeichel.

"He had a bad night but it still doesn't warrant that kind of treatment."

Thursday night was the latest in a string of perceived errors made by Schmeichel, following Joe Hugill's looping header for Kilmarnock last weekend and Jonathan Rowe's effort for Bologna last month.

Perhaps unsurprisingly, Celtic closed ranks around their embattled goalkeeper post-match.

"It's a tough evening for us," interim manager Martin O'Neill said. "On paper, 4-1 looks pretty poor.

"We were masters of our own downfall in many respects, conceded some poor goals.

"Kasper [Schmeichel] has made some great saves while I've been here. This is a collective, there's no doubt about that.

"We played Feyenoord in a game we probably had to win and he made an unbelievable save, otherwise we wouldn't be here playing tonight.

"I will think about all sorts of things [before the next game],...