Luke Shaw is undergoing scans on a groin injury sustained before Watford's second goal on Sunday.
The Manchester United full back was heavily criticised by Jose Mourinho for his role in Juan Zuniga's strike following the 3-1 defeat at Vicarage Road.
Mourinho accused Shaw of backing off Nordin Amrabat, allowing Watford to capitalise down United's left before static defending then saw Zuniga sweep past David De Gea unopposed.
It has transpired that the 21-year-old felt the full force of a 50-50 challenge earlier in the half and aggravated a groin problem he has been battling with.