The Red Devils were firmly in control in the first leg of their Europa League quarter-final after Henrikh Mkhitaryan gave them a first-half lead.
However, United missed a host of opportunities and were made to pay when the hosts struck back through Leander Dendoncker, prompting Mourinho to hit out at his attackers for failing to do their job properly.
"I think he was right in this matter," Ibrahimovic told Sky Sports News. "I wasn't feeling good in the sense of I was feeling tired but it is not an excuse because I still had to do much more and much better than I did.
"I speak for myself and it was not a good performance. We've played many games and I've played in almost all of them.
"But we have one month to go, we are still grinding, still pushing because we have the Europa League and the end of the Premier League. We fight for it."
The return leg against Anderlecht takes place at Old Trafford next Thursday.