Now he has returned, United's medical staff will ascertain the extent of the damage and how long he will be sidelined.
Jose Mourinho's squad has been hit by several injuries in midfield as they gear up to face Liverpool at Anfield on Saturday.
However, there was better news for United regarding the fitness of Lukaku as Martinez also confirmed that the striker is considered fit and available for selection.
That is set to rule him out of games against Liverpool (on Saturday), Benfica (Oct 19), Huddersfield (Oct 21) and Swansea (Oct 25).
The club are believed to be concerned that his running style may have been the cause of the injury, and are considering using a running coach in the hope of avoiding a recurrence.
He may not be a favourite among Manchester United fans, but few now argue with the effectiveness of Marouane Fellaini.
It seems very likely that Ander Herrera will play for Manchester United alongside Nemanja Matic at Anfield on Saturday lunchtime.
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