For context on Schoonmaker's production, only 10 D1 teams ran the ball more (i.e., had a lower percentage of pass plays) than Michigan last year. That includes the three service academies. An offense like that isn't going to put up big receiving numbers. In their first three games, they split the (meager) targets across three TEs, before Schoon became the primary guy and started putting up decent numbers.
(Though it's fair to point out that Notre Dame didn't throw the ball very much either, and Mayer still had a lot of production.)