MIAMI -- Manchester United manager Jose Mourinho has revealed Romelu Lukaku has agreed to cut short his holiday in a bid to be available for the start of the season while admitting he "doesn't know" when Anthony Martial will return to training.

Lukaku, who scored 27 goals last season, played for Belgium against England in the World Cup third-place playoff on July 14 before spending time on holiday in the United States.

He was due to return to Carrington on Aug. 5 -- just five days before the Premier League season starts against Leicester at Old Trafford -- but Mourinho has revealed the 25-year-old, along with Marcus Rashford and Phil Jones, has agreed to cut short his break.

Even with key players returning early, Mourinho says he is expecting the first three games of the season against Leicester, Brighton and Tottenham will be "tough".

"Rashford, Jones and Lukaku, they gave to the group and to the team three days of their holidays so they're coming back three days early to try and be available for the team," he said after the 2-1 win over Real Madrid at Hard Rock Stadium on Tuesday.

"With the fantastic spirit we have in the majority of the players, I repeat, in the majority of the players, we have a fantastic spirit, we go with everything we have to the Premier League.

"Let's look at it from a positive side and wait for [Victor] Lindelof, Rashford, Lukaku and Jones, which are going to be four important players for us, not prepared but they will be back and that's the spirit we need because I think our first two or three weeks in the Premier League -- Leicester, Brighton, Tottenham -- I think these two weeks are going to be really hard for us because we didn't prepare for that and that's what it is."

Romelu Lukaku is returning from his World Cup break early in a bid to start the Premier League season on time. Photo by Marc Atkins/Offside/Getty Images

Lindelof has already returned to Carrington following the World Cup while Jesse Lingard, Ashley Young, Marouane Fellaini and Paul Pogba -- not mentioned by Mourinho as part of the group returning early -- are still on holiday.

Lingard and Rashford, on a break together in Miami, were both in the stands for the victory over Real Madrid on Tuesday.

Meanwhile, Mourinho says he is unsure when Martial, who wants a transfer this summer, will re-start training after leaving United's U.S. training base last week for the birth of his son.

Following the defeat to Liverpool in Ann Arbor, Mourinho questioned why the 22-year-old had not returned to the tour and asked again Tuesday night, the Portuguese coach was unable to shed any light on when he might be back.

Mourinho and his players left the United States immediately after the victory over the Champions League holders and will land in Manchester on Wednesday morning.

They are due to train at Carrington on Thursday ahead of their last preseason friendly against Bayern Munich in Germany on Sunday, but Mourinho appeared unsure about whether Martial would take part in the session.

Sources have told ESPN FC the forward, signed from Monaco for £36 million in 2015, wants to leave this summer after growing disillusioned with life at Old Trafford during the second half of last season.