Smalling limped off in the first half of United's 2-1 win over Southampton on Monday night.
Smalling limped off in the first of United's 2-1 win over Southampton at the weekend, adding to Louis van Gaal's defensive injury worries.
He was replaced by Jonny Evans who returned from injury on Monday night, as Smalling joins Phil Jones and Luke Shaw amongst the defenders in the treatment room.
The Telegraph's report states that Smalling had a scan on his groin, which showed a minor strain, but it keeps him out of this weekend's huge clash with Liverpool at Old Trafford, as well as the trip to Villa Park the next weekend.
He is in line for a return to action against Newcastle United at Old Trafford on Boxing Day at the earliest, but there is some good news in that Rafael Da Silva is now available following a spell on the sidelines.
One positive is that it won't be too long until Smalling is available again as injuries continue to be a challenge for the club this season.
United will go in search for their six successive Premier League win on Sunday as they look to consolidate their position in third place in the table.
But they could end up with Paddy McNair or Tyler Blackett being in contention for a place alongside Evans on Sunday given Smalling's injury, depending on how van Gaal decides to set up his team for the visit of one of their fiercest rivals.