Europe aside, Celtic's season had been rattling along relatively well.

They had bounced back from League Cup and league losses to Rangers with a solid win at Motherwell, a resounding 7-1 victory at home to last season's runners-up Hearts, and a hard-fought 2-1 win from behind at Hibernian.

Wednesday's 2-2 draw with Dundee United turned out to be a missed opportunity to extend their lead at the top and they now face the prospect of Rangers and, indeed, Hearts getting closer should they win their games in hand.

Besiktas in the Europa League are next up, a home game Celtic really need to win to stand any chance of progress in that competition after starting with three defeats.

Former Celtic goalkeeper Pat Bonner said of O'Neill on BBC Radio Scotland that it's been "a brilliant weekend for Rangers", who won their League Cup semi-final with Aberdeen on Saturday.

"They're in the cup final and Celtic getting beat allows them, when they play their game, their extra game, to close the gap.

"That then puts huge pressure on to Celtic with the European game coming up now in midweek also. That is going to be tough for them to bounce back from this."