The Bucs made the announcement Tuesday, placing the eighth-year pro on injured reserve. He was hurt during Sunday's 33-20 loss to the Buffalo Bills.
Losing Phillips weakens a struggling secondary that was already playing without starting safety Tanard Jackson, who was suspended for the first four games of the regular season for violating the NFL substance abuse policy.
Phillips entered training camp as the projected starter at weakside linebacker, where the Bucs were replacing Derrick Brooks, who was released during the offseason.
Phillips returned to safety when Jackson was suspended. He had 11 tackles in Tampa Bay's first two games.