Zack Oliver is the primary back-up,but redshirt freshman Matt Alvitti is probably the starter-in-waiting. Alvitti was a Rivals 4-star in high school with offers from Illinois, Indiana, Notre Dame, Michigan State, Nebraska, and Purdue. He was a highly-ranked dual-threat QB from Park Ridge, Illinois (suburban Chicago). My guess is Fitzgerald will want to get a dual-threat guy back under center. They've missed that element with Siemian. Still stinks for Siemian.
PS--Northwestern also has true freshman Clayton Thorson who the Gophers recruited. He was also ranked quite highly coming out of high school (another suburban Chicago kid). I doubt they will burn Thorson's redshirt, but he'll be in the mix for next year.