Beilein was quoted recently saying there are only two players he sees as point guards on the roster, Garland and Delly. He also said Delly has the best grasp of the offense on the team.
Delly isn't getting any quicker and his shot will always be something he needs to monitor, but I don't think he is in trouble losing the backup point guard role. Knight was a combo guard who was supposed to learn how to run an offense, but at age 27 I don't think it's happening for him. If Delly's thumb doesn't allow him to play, that is another issue altogether.