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.