Martial, 22, left the club's tour of the United States last week.

He has been linked with a move away from Old Trafford and it is understood manager Jose Mourinho is willing to sell the France forward.

He said on social media his daughter was fine and "for the mom it was harder but thanks to God she's better now".

"Sorry but my family will always come first," he added.

Martial has had his first-team opportunities limited by the arrival of Alexis Sanchez in January.

He scored 11 goals in 45 appearances last season but failed to make the France World Cup squad.

The forward, who arrived from Monaco for £36m in September 2015, has a year left on his contract, although United have the option to extend by another season.