Rangers boss Mark Warburton centred on a refereeing decision when asked to reflect on his side's 5-1 derby defeat to Celtic.

The Ibrox boss endured a day to forget with Philippe Senderos sent off with the score at 3-1 but the Englishman was irked by Willie Collum's decision not to punish Mikael Lustig for a foul on Barrie McKay in the 77th minute.

Warburton mistakenly believed the Swedish defender had already been shown a yellow card and felt a caution and a subsequent red should have been forthcoming for the tackle on winger McKay.

Rangers faced an uphill battle to take anything from the game at that stage but Warburton insisted the decision was a key moment in the match with assistant manager David Weir sent to the stands for his protests.

"The sending off I can't defend, it's a red card," he said. "The frustration for us came at 3-1 with an uphill struggle we've felt Barrie McKay was taken out by a player on a yellow card.

"That's why David Weir was sent to the stands. We never, ever want to see players sent off, Philippe had to go though, absolutely.

"But we felt on the far side Barrie McKay was clear as day taken out and David Weir said he's got to go and the referee turned around and said 'no, play on.'

"At that point we were very frustrated and we had a choice to make. The choice I made was to have a go and try and take something from the game.

"That was the decision I made, no-one else. I made the choice to go for it a little bit.

"We put on Harry [Forrester] and [Martyn Waghorn], tried to be positive and get a second goal at 3-2 and try and get something out of the game. That was the thinking."