The Denmark midfielder sustained a knock at the start of the second half and was then caught on the ankle by Reading forward Andy Carroll, who saw red in the second half of the match for two other bookable offences.
After the match, footage was shared on social media of Eriksen hobbling away from Old Trafford to give Erik ten Hag an injury headache, though the manager said he must wait before he knows the extent of the problem.
"It's always difficult to say in this moment so short after the game has finished," Ten Hag said in his post-match press conference.
"But it's an ankle (problem). We have to see what is the diagnosis, so it costs us a minimum of 24 hours before we know that and then I can say more."
Eriksen has established himself as a key part of United's revamped midfield since his summer arrival from Brentford.
United are next in action when they host Nottingham Forest in Wednesday's second leg of the Carabao Cup semi-final, having taken a 3-0 lead in the first leg, before the visit of Crystal Palace in the Premier League on Saturday.