Swansea striker Wilfried Bony had to be replaced after picking up an injury just 47 seconds into their 3-1 defeat at Everton on Monday Night Football.
Bony was involved in an innocuous collision with former Swansea defender Ashley Williams on the halfway line and was unable to take any further part.
Swansea head coach Paul Clement told Sky Sports after the game: "He's injured his hamstring. I didn't see how he did it but he signalled to the bench that he had done some damage. We don't know the extent of it yet".
It is the second consecutive game in which Bony has failed to see out the 90 minutes, being replaced by Leroy Fer in the 74th minute of Swansea's last game against his former club Manchester City which they lost 4-0.
Leroy Fer had given Swansea a 35th-minute lead at Goodison Park but goals from Dominic Calvert-Lewin, Gylfi Sigurdsson and Wayne Rooney consigned Clement's side to defeat.
Swansea have now lost six consecutive away games in the league, last picking up a point in their 0-0 draw with Tottenham at Wembley on September 16.
Swansea remain two points adrift at the foot of the Premier League and are four points from safety.