Antonio Valencia suffered a hamstring injury in Manchester United's 2-1 victory over West Brom on Sunday, but further details about how long the 32-year-old will be out for have still not been made available to José Mourinho.
Despite a late rally from West Brom which saw Gareth Barry score with just 10 minutes remaining, first-half goals from Romelu Lukaku and Jesse Lingard proved to be enough for Manchester United to claim all three points at the Hawthorns.
"This is what I call a December injury. It is an injury of accumulation of fatigue," Mourinho confirmed to the club website. "It is a muscular one. Hamstring. Big, small, medium, I don't know.
"Let's wait," he added when asked about how long Valencia would be out for. "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."
United's fringe players will have the chance to impress their manager on Wednesday when the club travel to face Bristol City in the quarter-final of the Carabao Cup.
The Red Devils then face three more Premier League matches before the turn of the year, with a trip to Leicester swiftly followed by matches against Burnley and Southampton at Old Trafford.