Ibrahimovic has already made 44 appearances and scored 28 goals as United reached the latter stages of three cup competitions and challenged for a place in the Premier League's top four.
Jose Mourinho's side won the League Cup thanks to Ibrahimovic's heroics, but they have been unable to keep pace with league leaders Chelsea.
The 35-year-old Swedish striker believes Chelsea have benefited from their lack of European football, while United have been involved in the Europa League.
"We have one month to go, it's many games we play," Ibrahimovic said in quotes reported by the Daily Telegraph ahead of Sunday's clash with Chelsea.
"I have been playing almost all the games. But we are still grinding, we are still pushing because we have this Europa League and the end of the Premier League. We fight for it."
Asked about Chelsea, he added: "Overall a good team and then of course they are playing once a week which makes a difference also because at the end of the season, I don't know how many games we have in total but I think we have much more than them.
"They haven't been playing in Europe this season and they went out from the League Cup early. But they have a great team, or else they would not be number one."