City's second straight draw means United are nine points clear of the second-placed champions with 13 matches left.
A possible foul on Edin Dzeko in the build-up to Liverpool's first equaliser left Mancini fuming.
"Before the first goal it was a big foul. If the foul happened against us it was probably a red card for Vinny Kompany," Mancini told Sky in reference to Kompany's red card at Arsenal last month that was later rescinded.
"Instead the referee and linesman didn't see anything."
Daniel Sturridge grabbed Liverpool's deserved 29th minute equaliser but Mancini reckoned the visitors should have kicked the ball out with Dzeko down on the turf in the other half.
The Bosnian in truth was as much frustrated at not getting the foul as he was injured as he sat on the ground.
"Probably (they should kick it out) because after this there was 15 seconds. I think that the rules are the same for everyone always. The linesman was perfect for 10 minutes - all the fouls for Liverpool every time, every time with the flag but he didn't see this," Mancini murmured.
Former City forward Sturridge, doubtful for England's friendly with Brazil on Wednesday with thigh trouble and booked for diving in an otherwise excellent display, begged to differ.
"From our viewpoint the ref said to play on, he said not to kick the ball out. He said for us it is not our job to kick the ball out, if the referee says to kick it out we kick it out and do that," he said.
"Sometimes you do fair play things for the opposition and sometimes they don't give you the same."
Sturridge had equalised Dzeko's opener but it was the other way round in the second half as Steven Gerrard's fine effort made it 2-1 before Aguero scored from an acute angle after 78 minutes when keeper Pepe Reina rashly charged out of the box.
The Argentina striker's bit of brilliance was only partly appreciated by tough Italian taskmaster Mancini.
"Few players I think (can hit the target like that), he scored a fantastic goal but I think that Sergio and Edin should play better than tonight," he remarked.
Today we didn't play well, in the last month we play well all the games but today we were so nervous, I don't know why."
Despite his nitpicking, Mancini has not given up hope in the title race after they also overhauled United last term.
"Thirteen games to the end, it is long. I think we can recover nine points," he said.