The 23-year-old Argentine striker's absence will be a blow to fifth-placed Inter, who need to finsh in the top three if they want to secure a place in next season's Champions League competition. They trail third-placed Roma by five points.
Icardi sustained the injury during last Saturday's 2-1 win over Bologna and left the field 10 minutes into the game.
"I don't know if the match will decide (the Champions League spots) because there are still lots of points to play for and a number of the top sides have to meet each other," Mancini told a news conference.
"It will, however, definitely be important to win the match because with three points we could cut the gap."
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(Reporting by Silvia Recchimuzzi in Gdynia, editing by Pritha Sarkar)
