Jose Mourinho pays a visit to Belgium in search for answers over Romelu Lukaku's Manchester United form

Jose Mourinho has revealed he travelled to Belgium during the international break in search for answers about Romelu Lukaku’s worsening goal drought for Manchester United.

The United manager also denied Alexis Sanchez has told him he was unhappy at Old Trafford or wanted to quit the club but reiterated that he would not stand in the way of any disgruntled player who wished to leave amid continuing doubts about the Chilean’s future.

Mourinho’s concerns about his attack are not limited to just Lukaku and Sanchez, though. In a separate interview, the Portuguese claimed Marcus Rashford, Anthony Martial and Jesse Lingard - as well as defender Luke Shaw - still lack the “maturity” and “character, personality … natural aggression” needed to realise their “great potential”.

Despite scoring five goals in his past four appearances for Belgium, Lukaku has not scored in his past 10 matches for United and was dropped for the 2-1 win over Everton last month.   

Mourinho said he went to Brussels last week in order to try to better understand why Lukaku was flourishing for his country but not his club and, although he declined an invitation to meet in person with Belgium coach Roberto Martinez, he said they spoke on the phone. Lukaku had reported for international duty but sat out the games against Iceland, which Mourinho watched, and Switzerland in order to work on his fitness after a hamstring problem.

Sanchez's struggle for form is even more pronounced than Lukaku's, but Mourinho insists the player is staying at Old Trafford Credit:  Getty Images 

“I went, fundamentally, to be with Lukaku, to feel him, to see his situation, his injury,” said Mourinho, who has admitted previously that the striker’s struggles were becoming a worry. “Roberto invited me to meet him, which I decided not to because I don’t like to do disturb people before matches, but I was with Roberto on the phone trying to feel, trying to understand the problem and evolution of the problem.”

Sanchez’s problems in front of the net are even more pronounced, with just one goal all season, and Mourinho took a question about the Chile forward’s future in his stride when asked if the player could depart in January. “I don’t think so because he never told me that he wants to leave or isn’t happy to stay,” Mourinho said, before adding that his reluctance to hold an unhappy player against his will was “a personal thing related to my philosophy”.

Pogba and Martial are back again for Manchester United this weekend Credit: Getty Images

Mourinho added: “He [Sanchez] is improving like the team is improving, and when he plays he giving us his personality, his desire to play and to produce for the team and trying to adapt to how we play, and vice versa. Can he do better? I think we can all do better. I can do better and he can do better and all the other players can also do better.”

Mourinho confirmed Paul Pogba and Martial were available to face Crystal Palace on Saturday after injuries but that defender Eric Bailly would again be omitted from the squad. Mourinho said he had come to favour Victor Lindelof and Chris Smalling as a centre-half partnership because both were less injury prone that Bailly, Phil Jones and Marcos Rojo.

He also suggested Smalling was a better partner for Lindelof than Bailly because he was taller than the Ivorian and “very good in the air, dominant in the air” and provided “some balance” to Lindelof’s ability to bring the ball out from the back.

Meanwhile, United have been fined 8,000 euros for three pitch invasions during their Champions League match against Juventus at Old Trafford last month.