The early indications from Swansea's medics were that Michu had not suffered ligament damage, but the Spaniard left the Madejski Stadium with his ankle heavily strapped.
Michu is the joint top scorer in the league with 13 goals and is now very doubtful to play against Fulham at Craven Cottage on Saturday.
Swans manager Michael Laudrup said: "His ankle is twice as big now. We don't know how bad it is.
"The doctors and the physios say it shouldn't be the ligaments, which is the most important thing.
"The next question is `will he play on Saturday?' I don't know. We will have to wait 24 hours.
"Hopefully it is not too bad. It is not a muscle injury but still we have to be careful. He has to be 100 per cent.
"He is one of the top scorers in the league, definitely our top scorer. He has played nearly every match this season.
"Even if he is the most important player, we have to deal with it (if he is out). If you rely so much on one or two players it is not good for any team."