This is the NFL, you can't just throw a guy into a position he's never played before and expect it to work out.
It would be a long, difficult process for him, with no promises that he's ever even good enough to start. It's fundamentally different from moving someone from CB to FS, or DT to DE...there are skills and reads and techniques at LB that a defensive line player has simply never ever been asked to make.
If they're considering this, his chances of living up to his draft status are slim to none, and slim is leaving town