Manchester United manager Jose Mourinho says he faces a "difficult" summer transfer window as he tries to bridge the gap with Manchester City.

The champions are 19 points clear of second-place United ahead of the final round of Premier League fixtures on Sunday. Four days later, the window opens and Mourinho is set to add to his squad as he looks to mount a serious Premier League title challenge next season.

A new central midfielder tops his list of priorities but Mourinho has warned that summer recruitment will be tough with so much ground to make up.

"The difference of points tell a lot," he said. "I always think that in the championships the truth is always there. In knockout competitions sometimes, in a lucky or unlucky moment, the truth is not always there.

"Over 38 games the reality is the numbers. And the reality of the numbers is between second and fifth, the difference is three, four or five points. The difference between the first and the second is a difference of 19 points. That's the reality of the numbers.

"Can we close that gap in one season with a very difficult market, an absolutely difficult market? We are going to try."

City were crowned champions with five games to spare last month after United were beaten by West Brom. Pep Guardiola's side have scored 105 goals and can reach 100 points with a win at Southampton on Sunday.

Jose Mourinho is coming to the end of his second season as Manchester United manager. Nick Potts/PA via AP

Mourinho, who will also have to appoint a new assistant following Rui Faria's departure, says the title was destined for the Etihad Stadium as early as two months into the season. While he insists he will not celebrate second place, the 55-year-old has praised the "professionalism" of his players in making sure the spot was secured with a draw against West Ham last Thursday.

"I'm happy because it was the position that was available since October or November," Mourinho said ahead of Watford's visit to Old Trafford on Sunday. "Two things could happen to a club like us: One would be 'I cannot be first and I'm not professional, I'm not committed, I have no pride and it's the same to finish second, third or fourth.' The other was 'this is a position that is available and we are going for it.'"

Finishing runners-up is United's highest league position since Sir Alex Ferguson retired in 2013 and May 19's FA Cup final vs. Chelsea means a trophy could yet be Old Trafford-bound this season.

"We played for honesty and professionalism and pride and we managed to finish second and that's very positive," Mourinho said. "If we beat Watford we can jump the wall of 80 points and in the two major domestic competitions of this country we are at least second best in both.

"We are the second best in the Premier League and one of the two best in the FA Cup. Is it enough to celebrate? No. Not for me. But we did our job the best we could and the boys can feel they did a positive Premier League, of course much better than last season."