Scott Sorensen's challenge at the NRL Judiciary has been unsuccessful.
The Panthers forward will now serve a three-match suspension for his shoulder charge on Nathan Brown.
Sorensen was sent to the sin bin for the shot in the 26-10 defeat to the Sea Eagles.
He could have accepted a two-week ban with an early guilty plea, but opted to contest the grading of the charge.
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It's a blow to an already-struggling Penrith side, who sit outright last on the ladder after eight rounds.
The reigning premiers have won two games and lost six.
Sorensen will be unavailable for their Magic Round clash with the Broncos, as well as matches against the Cowboys and Knights.
Isaiah Papali'i will likely be promoted to the starting side, with the likes of Daine Laurie and Casey McLean in contention to be added the bench from the reserves list.
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Sorensen joins a string of fellow NRL players to have received suspensions in Round 8.
Bulldogs duo Sitili Tupouniua and Joshua Curran copped multiple-game bans, with Latrell Mitchell, Jaimin Jolliffe and Brock Gray also hit with sideline spells.
In total, 16 games' worth of bans were issued across the most recent slate of matches.