Palmeiras say forward Gabriel Jesus has a groin injury but deny that it was caused by fatigue due to match overload.
The Brazilian teenager is playing on loan with his home club until the conclusion of the Brazil league's national championship in December after signing with Manchester City last month for a reported £27 million on a five-year deal.
He is being evaluated by Palmeiras doctors on Monday after being taken out on Sunday in the team's 0-0 draw against Gremio.
Jesus has played three matches in six days, seeing action in Brazil's World Cup qualifying win over Colombia and then rejoining Palmeiras the following day for the 2-1 victory over Sao Paulo before Sunday's draw against Gremio.
He also scored a brace in his Selecao debut on Sept. 1, helping Brazil to a 3-0 win over Ecuador in the South American World Cup qualifiers.
Palmeiras doctor Rubens Sampaio denied that fatigue could have caused the injury, for which Jesus will undergo testing on Monday.
"There were no signs of fatigue after those two days of work and he was feeling fine," Sampaio said. "He had an adductor injury, which is more related to the mechanics of his movement than to fatigue. It has nothing to do with the sequence of matches."
Palmeiras manager Cuca also defended the team's decision to play their teenage star the day after Selecao action.
"Sure, the critics are going to gossip and say that we put the kid in to play right away and now he is banged up," Cuca said. "But our Palmeiras trainer checked him and said he was at 100 percent. That is the kind of injury that could happen to anyone."