The Englishman was substituted off in the second-half at Old Trafford for Marcos Rojo.
Speaking after the game, Mourinho confirmed Smalling had a bad muscular feeling.
Mourinho on Smalling: "He had a bad muscular feeling. We were 3-0 up so we decided to change. Hopefully he can recover for the weekend."
— Manchester United (@ManUtd) October 20, 2016
The good thing about Man United’s current defensive options is that even without Smalling we’ve got Daley Blind and Eric Bailly as options and they would be an excellent partnership against Chelsea. However, that’s not to say Smalling being injured wouldn’t be a blow.
United play City three days after our trip to Stamford Bridge so if Smalling isn’t fit then Mourinho will likely have to look towards Timothy Fosu-Mensah or Axel Tuanzebe as a defensive option which would be a huge occasion for either player in the derby.
We’ll bring you updates on Smalling’s fitness in the next few days as soon as we have them.