Mathieu a doubt for Bilbao clash
The defender has recently returned from international duty with France, having played the full 90 minutes as Didier Deschamps' men drew 1-1 with Serbia in a friendly on Sunday.
Barca have revealed that their new signing from Valencia was forced to miss training on Wednesday due to a problem with his right ankle.
"The evolution of the injury will determine his availability for the rest of the week," read a statement on the Catalan giants' official website.
Mathieu has started both of Barca's league matches so far this season, playing his part in wins over Elche and Villarreal.
Sunday's appearance against Serbia represented the 30-year-old's third cap for his country.