Neither can you. But past performance is considered a predictor of the future. Crowell has made few, if any, game-changing plays, and what he has made are on the order of what Ellison has provided.
And again, Crowell had a bum knee. He would have contributed even less had he played, and ended-up contributing nothing because he decided he needed to have surgery after delaying it for so long. And that I don't need to predict.