Mourinho, who is back from a one-match suspension for kicking a water bottle in frustration during the 1-1 draw with West Ham United last weekend, said his objective was to win the league in his first season.
"In the league, we have not been getting the goals our performances deserve," he said.
"There is no doubt we are improving, though. We know where we want to go and our destination is to be champions. Maybe we cannot do it this year," Mourinho was quoted as saying by The Guardian on Friday.
"You can never tell in football but the gap might be already too large. I don't know when we will do it, maybe when I am not here, but that is what we are working towards and I think it will happen," he added.
He also spoke on the strategy of previous Manchester United manager Louis van Gaal, asserting that he has a totally different approach and the players need time to adapt.
"Sometimes when a manager takes over a new club the differences in approach are not significant," he said.
"In that case, all it needs is a little touch, just a fingerprint from the new man and you can get a lot from what was already there," Mourinho said.
"Here we are trying to do something completely different. We could probably get better results if we weren't trying to go in an opposite direction.
"I am not talking about tactical systems here. I am talking about the way this team wants to play and that is the most difficult thing in football to change," he added.
Mourinho reckoned that van Gaal was too defensive in style, with too much sideways and backwards passing rather than looking forward to attack between the lines.