Salas, who was inactive during Week 2, was acquired via trade from the Rams on Sept. 1. In exchange, the Patriots shipped St. Louis a seventh-round pick in 2015.
Rivera, 26, was signed to the Patriots' practice squad in November 2011 and found a spot on the 53-man roster this season. He has played a special teams role in each of the team's two regular-season contests.
The team added Hilliard leading up the regular-season opener and served in a situational fullback role. Hilliard played four of the team's 82 offensive snaps in Week 2.
In addition to the roster moves, the Patriots hosted seven players for a work out on Tuesday: defensive tackle Antonio Dixon, offensive tackle Tommie Draheim, defensive tackle Terrell McClain, offensive tackle Nick Mondek, wide receiver Kashif Moore, wide receiver Brian Robiskie and linebacker Nathan Stupar.