The midfielder has returned to Carrington to be assessed after he suffered a knee problem while on worldwide duty with Belgium in recent days.
Another Belgian, Marouane Fellaini, returned to Carrington with knee ligament damage and will miss the clash at Liverpool on Saturday lunchtime.
However, despite the disappointment of Fellaini's setback, Mourinho has insisted that his team will simply take the latest injury problem it its stride. 'At Anfield I've had great moments and I've had bad moments, ' he told Sky Sports.
Mourinho said: 'We may cry with the process that leads to injuries but we do not cry because of injuries'. "We have our objectives for the season, we know where we want to go and how we have to go".
Victory against Liverpool, who are without Sadio Mane after the forward was injured playing for Senegal, would mean United's best ever start to a Premier League season.
They are now unbeaten in all competitions and find themselves perched at the top of the Premier League table alongside arch rivals Manchester City on 19 points.
However, United chiefs have been impressed by Mourinho and are convinced he is the right man to finally fill the void left by Sir Alex Ferguson. Lukaku is Manchester United's top scorer this season having made a blistering start to life at Old Trafford with 7 goals in 7 games, while on the Road to Russian Federation he's scored 12 goals in 15 games.
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