Lionel Messi is suffering from bone bruising, the Ligue 1 club said on Tuesday, and will miss Paris St Germain's trip to Metz on Wednesday.
"Lionel Messi, following the knock received on his left knee, has undergone an MRI this morning which confirms the signs of bone contusion, a new assessment will be made in 48 hours," PSG said in a statement.
The Argentine forward, who joined the French club from Barcelona on a two-year contract during the close season, was replaced with 14 minutes left in PSG's 2-1 home victory over Olympique Lyonnais on Sunday.
"We saw that during the game he was checking his knee and made some gestures," coach Mauricio Pochettino told a news conference.
PSG lead the Ligue 1 standings with a maximum 18 points.
Messi played half an hour against Stade de Reims last month before starting the last two games, against Club Brugge in the Champions League and at home to Lyon.
He has yet to score for PSG but has hit the woodwork twice.