Bayern, who made a shaky start to the season and finished six points behind Girondins Bordeaux as they struggled through the group stage, have won their last 12 games in all competitions under temperamental Dutch coach Louis van Gaal.
Fiorentina, who qualified for the knockout stages at the expense of Liverpool, have lost momentum since the start of the year, taking only one point from their last five Serie A matches.
They were outclassed 2-0 by Sampdoria on Saturday, almost inevitably sparking talk of a crisis in the Italian media, and have dropped to 11th in the table in one of the most difficult moments in the long reign of coach Cesare Prandelli.
Some fans have even called for Prandelli to leave amid speculation he is wanted by Juventus and Italy after the World Cup.
The team appear to have been hit hard by the ban on Romania striker Adrian Mutu, who was provisionally suspended after the Italian Olympic Committee said he had tested positive for the appetite suppressant drug sibutramine after the 2-1 win over Bari on January 10, the team's last victory.
OPTIMISTIC
Fiorentina suffered another blow on Saturday when centre-back Alessandro Gamberini dislocated his shoulder and was ruled out of the game at the Allianz arena while Argentina forward Mario Santana also limped off.
However, the players remain optimistic.
"We're certain that match against Bayern has come at the right moment, it's the most important match in Prandelli's five-year reign," said midfielder Ricardo Montolivo.
Bayern are taking nothing for granted and, despite their recent run, van Gaal is proving hard to please.
Having publicly criticised the team following their 3-1 win at VfL Wolfsburg and 6-2 Cup win over SpVgg Fuerth, he was complaining again following Saturday's 3-1 win at home to Borussia Dortmund which left them level on points with Bundesliga leaders Bayer Leverkusen.
"We made a number of individual errors in defence, and that mustn't happen against Fiorentina in midweek. We also need much better organisation throughout the team,." he said.
Captain Mark van Bommel agreed.
"In our recent games, against Wolfsburg, Fuerth and today, we've let our opponents make a few chances.
"We've not been punished, because we've gone on to win. But in the Champions League, you can't afford any lapses and if you concede at home, it's even more difficult. The whole team has to do its job in this respect.
"If we all hare upfield and forget our defensive responsibilities and organisation, it'll be difficult. The number of chances Dortmund created today wasn't good at all."
