And the 27-year-old has also been ruled out of contention for England's two-Test tour to New Zealand next month.
Deacon, who made his Test debut against Samoa in 2005, has won eight caps.
He said: "The lads did really well to come back against Harlequins on Saturday, and I am gutted that I won't have a chance of playing in the semi-final.
Tigers forwards coach Richard Cockerill added: "It is obviously disappointing not to have Louis available for selection for Sunday.
"He has been an important player for us again this year, and it is a shame that he's not in contention for the match against Gloucester.
"It is 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."