Valencia has played in all 18 Premier League fixtures for the Red Devils this term and started his fifth match of December in the West Midlands, prompting Mourinho to blame the injury on the accumulation of games played by his stand-in captain.
Speaking in his post-match press conference, the Portuguese coach told reporters: "This is what I call a December injury. It is an injury of accumulation of fatigue. It is a muscular one. Hamstring. Big, small, medium, I don't know.
"Let's wait [before giving an estimated return date]. I think he is an experienced guy to not let it go to difficult limits. But it is an injury of fatigue. Everyone in the Premier League, especially the ones in more accumulation of cups, European cups, they are at risk."
Valencia has now made 24 appearances for United in all competitions this season, scoring twice.