Steven Gerrard was baffled by Bobby Madden's decision to award Celtic a penalty - but it didn't stop the Rangers boss savouring victory over their rivals.
The Premiership champions remain on course for a league and Scottish Cup double as they defeated their old foes 2-0 at Ibrox on Sunday.
An acrobatic Steven Davis effort and an own goal from Jonjoe Kenny put Rangers in command despite a slew of Celtic chances after the interval.
Ref Madden awarded the visitors a spot-kick for a Joe Aribo foul on Leigh Griffiths as time ticked on.
Gerrard watched Allan McGregor save from Odsonne Edouard but insists it was never a penalty.
He told Premier Sports: "It shouldn't have been a penalty. I thought the penalty was really soft.
"From Bobby's view, I find it hard how he's given that penalty.
"But again Allan McGregor has come big for us, he's pulled one out. I thought it was a real soft penalty.
"In a derby, in an Old Firm, normally you don't get those ones."
Overall, however, Gerrard was satisfied his side have returned to something approaching their best.
Celtic created chances aplenty but there was a sense the Ibrox men had another gear to find if required.
They will now face St Johnstone in the last eight next Saturday, and Gerrard said: "Really pleased, we were back close to our best.
"Individually and collectively, really pleased with the performances the boys gave today.
"Celtic played their part, it was a real good cup tie with chances at both ends.
"When you play against a good team like Celtic you always know they will have periods in the game but we sucked that up really well.
"We rode our luck at times but over the course of the 90 minutes I don't think anyone can take away from us that we deserved to win."
