The 27-year-old, who was voted Tigers' Players' Player of the Year last Wednesday having made 23 appearances this season, is also unavailable for England's summer tour to New Zealand.
"The lads did really well to come back against Quins on Saturday and I'm gutted that I won't have a chance of playing in the semi-final," said Deacon. "I'm massively disappointed to miss what would have been another opportunity with England as well. Hopefully, I won't have to wait too much longer for another chance."
Tigers forwards coach Richard Cockerill said: "It's obviously disappointing not to have Louis available for selection for Sunday.
"He's been an important player for us again this year and it's a shame that he's not in contention for the match against Gloucester. It's disappointing for him personally that he won't be able to tour with England as I think he's deserved to be part of the squad."