Manager Jose Mourinho had confirmed after the draw with West Ham on Thursday that Martial would start Sundays match against the Hornets, though many fans were stunned when the Frenchman was omitted entirely from the squad.
There have been plenty of rumours linking Martial with a move away from Old Trafford this summer, and his absence immediately prompted fears that fans might have seen the winger for the last time.
But MUTV confirmed ahead of the match that Martial picked up an injury in Saturdays training session, cutting his involvement short, and they do not want to risk him with the FA Cup final just a week away.
Speaking after hearing the news about Martials absence, John OShea said: Its a big concern because hes a match winner. Hes got outstanding quality.
Fingers crossed hell be OK, well have to wait and see.
Martial was seen at Old Trafford several hours ahead of kick-off before driving away, and it is suspected that he failed a late fitness test to be involved against Watford.
With Martial absent, Mourinho has drafted Ashley Young into a more advanced role on the wing in a front three along with Alexis Sanchez and Marcus Rashford.
More: Manchester United FC