Shaw suffered a concussion while playing for England against Spain during the international break last Saturday and was treated on the field for almost seven minutes.
Despite the injury, it was reported by Simon Stone of BBC Sport that the left-back was in the frame to face the Hornets on Saturday evening.
However, according to Samuel Luckhurst of the Manchester Evening News, the decision has been made to leave the player at home:
Luckhurst outlined what the absence of the England international will most likely mean for the Red Devils in the upcoming fixture:
On the eve of the game, United manager Jose Mourinho said it wouldn't be long before the left-back is back in action again, per Alex Richards of the Daily Mirror.
"Contrary to some news, to some opinions, by the protocol point of view and according to our doctor, he will be free to play," said the Portuguese. "The only situation we have to analyse is if you are going to play him when, during the week, he was not training with the team or if, even, he decides to play."
As Stone noted, while the guidelines from the Football Association say a player who has suffered a concussion shouldn't play for a fortnight, Shaw had satisfied seven criteria as part of his recovery, meaning he would've been able to feature if selected.
Simon Peach of the Press Association believes that the club have made the right decision:
When Shaw went down during the game for the Three Lions following a clash with Dani Carvajal, there were fears he was set for another stint on the sidelines. The 22-year-old broke his leg at the start of the 2015-16 term, and it's an injury he's struggled to fully recover from.
This season Shaw has shown signs he is approaching his best again, though, as he's been United's standout man in their first four games. On Saturday his raiding presence will be missed down the left flank, although it appears supporters won't have to wait for too long before seeing the defender in action again.