That is according to his former Porto player Dmitri Alenichev.
The Red Devils will almost certainly finish behind Manchester City this season.
But Alenichev, who won the Champions League under Mourinho at Porto, has backed is former manager to bring title glory back to Old Trafford.
"If I am honest, yes, I think [Mourinho] will stay at United for at least another three years. He is the kind of manager that Manchester United need, he is indispensable to them.
"Currently Manchester City are outdoing every team in the Premier League. But I think if Mourinho stays – the club hierarchy can decide that – if he stays, then even as soon as next year Man United have a real chance of becoming Premier League champions."
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