Liverpool captain Steven Gerrard issued a frank apology on Sunday after being sent off during his side's 2-1 Premier League defeat at home to arch rivals Manchester United.
The 34-year-old, who will join the Los Angeles Galaxy at the end of the season, came on at half-time with Liverpool trailing 1-0, only to be sent off after 43 seconds for a crude stamp on United midfielder Ander Herrera.
"I don't know what caused it. I don't know. Probably just a reaction to the initial tackle," Gerrard told Sky Sports.
"I shouldn't say more about it, really. I've just come out here to apologise to the dressing room and supporters.
"I need to accept it. The decision was right. I've let down my team-mates and the supporters. I take full responsibility.
"I've been in the game long enough (to know what happens) when you do something like that. I take full responsibility."
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 Liverpool goalkeeper Simon Mignolet.
Source: AFP