The ever present full-back had to be substituted with the injury in the 66th minute and was replaced by Marcos Rojo as the Reds got the important win against the Baggies with the goals being scored by Romelu Lukaku and Jesse Lingard.
Mourinho reveals that Valencia suffered a "December injury" against the Baggies
Valencia has been a vital part of Mourinho's side this season, playing in every single Premier League game but he had to come off injured on Sunday and the United boss feels that the injury is down to the amount of matches that the full back has played in a short period of time.
Speaking at his post-match press conference, Mourinho said that the injury Valencia suffered is "what I call a December injury" which is the kind of injury that happens with an "accumlation of fatigue" but for now "I don't know" the full extent of the injury but it is the "hamstring" that is the problem.
When asked about how long he felt Valencia could be out for, Mourinho said "let's wait" as he is an "expereinced guy to not let it go to difficult limits" so he knew to come off the pitch when he felt something "but it is an injury of fatigue" and every player that has played in lots of games this season "are at risk" of suffering the same injury.
Mourinho now has a selection headache with key players missing at such a key time
It would be a massive blow for United if Valencia is out for a number of weeks given how important he has been to the team this season as he has made 24 appearances in all competitions, scoring twice in wins against Everton and Arsenal.
The injury also gives Mourinho another headache given that he has already lost centre-back Eric Bailly for the next three months due to an ankle injury and with so many games coming up the United boss is going to need his full squad to cover for these big losses.