The 33-year-old replaced Arturo Vidal in the 69th minute of Chile's 6-0 friendly victory over Iraq but picked up an injury during his short time on the field.
The problem could keep Pizarro out of Fiorentina's upcoming UEFA Europa League qualifying clash with Grasshopper and, as a result, he has expressed regret over reporting for international duty.
"I shouldn’t have played," he said. "I wasn’t totally fit, but I wanted to feature and asked the coach to use me.
"I’ve picked up a muscle strain and I now fear a tear. I really hope that is not the case seeing as the official games for Fiorentina are on the horizon."
Pizarro made 28 starts for Fiorentina in Serie A last season, helping the club to a fourth-placed finish in Serie A.
The Chilean has also previously enjoyed spells at Inter, Roma and Manchester City.
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