The Red Devils will be looking to improve on their sixth-place finish in their first season under the management of Mourinho.
The Europa League winners have signed Benfica defender Victor Lindelof, Everton striker Romelu Lukaku and Chelsea midfielder Nemanja Matic in an attempt to close the gap on their title rivals.
However, there is still a belief that Manchester United are lacking pace in wide areas as the Red Devils continue to be linked with a move to bring in Inter Milan midfielder Ivan Perisic.
Real Madrid forward Gareth Bale is another name that has been linked with a potential switch to Old Trafford amid concerns about his long-term future in the Spanish capital.
Former Chelsea and Scotland winger Nevin has claimed that Manchester United would be genuine title contenders alongside Manchester City if Bale moves to Old Trafford before deadline day.
Nevin told Irish radio station Newstalk: “Manchester United have good players who can create but if they got Gareth Bale, with his pace, power and ability, that would be the only player that would make me think they could take on Man City.
“Other than that, they are a distance behind. They will be good to watch but I wouldn’t put money on them to win the title.”
Bale moved to Real Madrid in an £85.3m deal from Tottenham Hotspur in the 2013 summer transfer window.
Manchester United won the EFL Cup and the Europa League in their first season under Mourinho.
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