The 33-year-old sustained the injury during Chile's 6-0 friendly victory over Iraq and faces tests to find out the severity of the problem and whether or not he has suffered a tear.
Pizarro, who came on as a 69th-minute substitute for Arturo Vidal, claimed he was not fully fit ahead of the match, and he told Gazzetta: "I suffered a muscle injury, I was not well and did not have to play.
"But it's my fault: I asked to take the field. Now I'm afraid that it may be a tear."
The Former Manchester City and Roma midfielder added on Eurosport.com: "I really hope that (a tear) is not the case seeing as the official games for Fiorentina are on the horizon."