"Ribery has a slight problem," van Gaal, whose side have a 2-1 advantage from the first leg, told a news conference without giving the nature of the injury.
"I think he can play but we'll see tomorrow."
Ribery did not start for the Bundesliga leaders against Cologne on Saturday because of the problem, coming on in the last third of the 1-1 draw.
Van Gaal said fellow midfielder Arjen Robben should be available after missing the Cologne match with flu.
The Dutchman's biggest injury problems are in defence.
Martin Demichelis is out after suffering facial fractures playing for Argentina in last week's 1-0 win over Germany.
Diego Contento cannot play either because of a cut knee, which means 17-year-old Austrian midfielder David Alaba may be used in the backline.
But van Gaal was not spooked by these problems for the clash against an off-form Serie A outfit, with his side sure of reaching the quarter-finals if they extend their 18-match unbeaten run.
"Naturally, it (the situation in defence) is a disadvantage," he said. "But in training the players play in different positions and they know them, so I'm not afraid if there are other players on the field instead of Demichelis and Contento.
"During the course of the season, I've shown faith in six young players. If they didn't have the right qualities, they wouldn't play."
Fiorentina coach Cesare Prandelli, whose side have lost seven of their last 10 games in all competitions, wants the team to put behind them any rancour at the run of bad luck with refereeing decisions that started in the first leg, which Bayern won thanks to a late Miroslav Klose goal scored from an offside position.
"We shouldn't feel victims of the refereeing errors that, obviously, have penalised us," he said.
"It would be an own goal. If we let ourselves be conditioned by these errors, it would mean we are not mature."
Fiorentina midfielder Massimo Gobbi is suspended after his red card in the first leg. They are also without much-missed striker Adrian Mutu, who has been provisionally suspended after failing two drugs tests in January.
