Manchester City should discover the full extent of Kevin de Bruyne's hamstring injury by Wednesday.
The Belgium midfielder, 25, pulled up in the 81st minute of Saturday's 3-1 win over Swansea at Liberty Stadium.
Reports have already emerged saying De Bruyne will be out for up to four weeks, but it is understood he is yet to complete medical tests.
City manager Pep Guardiola said after the game that he did not know how bad the injury was.
"It looks like it was muscular, but I am not a doctor. It is a pity because he is an important player," Guardiola said.
"That's why we have to have a big squad as everyone will help us."
City's next game is a Champions League trip to Celtic on Wednesday (19:45 BST).
Saturday's victory meant they maintained their perfect start under Guardiola, with Sergio Aguero scoring twice and Raheem Sterling adding a third.
They are four points clear of nearest rivals Tottenham after six league games.
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