Bhoys boss Ange Postecoglou made a number of changes to his team for the Scottish Premiership visit to St Mirren on Sunday.
He saw his side stutter from the start and concede a goal to a Mark O’Hara header just two minutes before the half time interval.
Postecoglou brought on Reo Hatate and Jota at the break, but saw St Mirren score again in the 53rd minute.
He called for Sead Haksabanovic and Georgios Giakoumakis shortly after, and Matt O’Riley in the 74th minute, but Celtic could not breach the St Mirren backline and slipped to a 2-0 loss.
It was Celtic’s first loss in the Scottish Premiership in almost a year and Bhoys legend Bonner admits he is shocked.
He also acknowledged that Celtic failed to properly test the St Mirren goalkeeper and were generally flat in the game.
“I’m shocked in a way, because of the performances Celtic have put in all season – today they were flat”, Bonner said on BBC Sportsound.
“Normally when Celtic’s opposition get a result like this, you’re talking about the goalkeeper having a tremendous game, save after save – that was not the case today.
“Trevor Carson didn’t have to do an awful lot.”
Celtic now have to wait until after the international break to put things right, with their next fixture coming on 1st October against Motherwell.