Mourinho also says his old club should take the Europa League seriously.
Asked by Sky Sports if he thought United could win the title last season Mourinho said: "No, when I said finishing second in the previous season, was one of my biggest achievement, I was not having fun with the words, I was just telling you what I was feeling.
"So last season I didn't think we did enough in the transfer market to compete for the title. I hope they feel they can [win it this time] because it will be an important motivation for them."
When asked what would be success for United this season, he said: "As a start, winning the Europa League. I think this club with this history, and when you go to a competition that doesn't belong to you, you have to win.
"When you go to the Premier League, the dressing room must feel they can win the title.
"If they don't feel it they are in trouble. But I think if they finish in the top four that would be a good achievement."