He asked the staff for the move, since he's 6th on the RB Depth Chart, plus CB Jonathan Jones is out with some unknown surgery that'll probably keep him out for the season.
This move could mean a better future (long term) for him plus we will get to witness that speed.
IMO it's not a waste for speedy guys like Grant or even Marshall to have that experience and OMac is a good example. Darron Thomas could even be another if the QB had prepared for a time as a corner instead of just getting a look as a WR with a couple of NFL teams. At 6'3" 215, he might be making five times what he is currently earning in Canada as a QB.
To me, putting an ego aside is a sign of maturity. Kudos to Rudy and his future.