Mourinho faces his former club tonight in the Europa League and commented on the potential of Ten Hag's team.
"Maybe you English guys think I'm bluffing, playing mind games," he said at his Europa League media conference.
"I'm not playing anything. They have a better team than results are showing. I studied them the maximum I could. We watched everything in the Premier League, (and the European games against) Twente and Porto – there is work there.
"They will succeed sooner or later. Hopefully sooner and, before one day, I go back to the Premier League and they become my opponents. At this moment, they are just my opponents for one match.
"I wish the best to Manchester United since the moment I left. I left with a good feeling for the club and its fans and I want the best for them.
"If things aren't going amazingly well for them, it’s not something that makes me happy. But I have no time, it doesn’t make sense for me to be thinking about what happened, what didn't happen.
"What happened, for sure, is that they now keep faith in the coach, supporting the coach, who is staying season after season. That means stability and means trust, giving him conditions to keep his job."
Mourinho also rates United as among favourites for the title.
"If I have to say now which are the two biggest candidates to win the Europa League, I think it's easy," Jose declared. "Manchester United and Tottenham.
"The Premier League is a different level of quality, intensity, pace, tactical culture – a different level of everything. Tomorrow, I think we are going to play against one of the two strongest teams.
"I think they will come with maximum strength," he said of United. "I don’t think they are going to rotate players before next weekend’s match (at West Ham United). They come full strength... that’s what I’d do."