The club captain was sent off just 38 seconds after his half-time introduction against arch rivals United.
The former England skipper, who will end his long association with the Reds this summer when he moves to Los Angeles Galaxy, had just come on as a substitute only to be dismissed for a crude stamp on Ander Herrera.
Referee Martin Atkinson barely hesitated as he brandished the red card in the direction of Gerrard, which was his sixth top flight red card for the Merseysiders.
“I just need to accept it,” the 34-year-old told Sky Sports. “The decision was right. I’ve let my team-mates and manager down today and even more importantly I’ve let all the supporters down so I take full responsibility for my actions.
“I tried to jump his tackle and I’ve seen his studs and I’ve reacted wrong.
“I’ve been in the game long enough (to know what happens) when you do something like that and especially at the time of the game – after half-time with 45 to play – with a great opportunity to get back into the game, I take full responsibility for today’s result.”
Juan Mata scored twice for United, with Daniel Sturridge replying for Liverpool, as Louis van Gaal’s side established a five-point lead over their rivals in the battle for Champions League qualification.
United striker Wayne Rooney also saw a stoppage-time penalty saved by goalkeeper Simon Mignolet.