Grealish was substituted with a groin problem in the first half of the holders' 3-1 victory in the last 16 first leg in Denmark on Tuesday.
City playmaker Bernardo Silva also came off with an ankle knock, but it was Grealish's injury that caused the most concern for City boss Guardiola.
"He wanted to continue but we didn't want to make the damage even worse. We'll make tests on Wednesday," Guardiola said.
Grealish was making his first start in six games in what has been a difficult campaign for the England star.
The winger had a spell out with a dead leg earlier in the season and has been kept out of the side by the fine form of Jeremy Doku and Phil Foden.
Another lay-off would be a blow for City as they chase another treble.
Silva could be doubtful for Saturday's Premier League clash with Chelsea, while it has also emerged that Josko Gvardiol and Mateo Kovacic have been nursing knocks.
Neither featured in Denmark and although Kovacic should be back in contention for the weekend, Gvardiol faces two to three weeks out with an ankle injury.
