Rosen left last week's game at Washington with what was believed to be a finger injury, but the Los Angeles Times reported Friday that Rosen also has a concussion and did not travel with the team.
With the star junior unavailable, redshirt freshman Devon Modster will make his first career start. Modster completed 7 of 12 passes in relief of Rosen at Washington for 77 yards, with a touchdown and no interceptions. He has attempted 15 passes this season.
The game is important for each teams' pursuit of bowl eligibility. The Bruins (4-4, 2-3 Pac-12) and Utes (1-4, 4-4) both need two wins in their final four games to reach the postseason.