1st - a redshirt still is a scholly. Signing Lamb means letting Roberts out of his LOI or kicking someone off the team.
2nd, Gene Keady is great. He'd be a consultant, not an every day kind of guy. Lavin started under Keady, whose coaching tree includes Bruce Weber, Matt Painter (I think) and Chris Lowery (I think). Good guys with good defensive principles (and sometimes offensive as well).. It would be easier for Lavin to lean on a guy he knows and who taught him some of what he knows, than Carnesecca, who might be wise but has shown no inclination to help with the coaching, & may have different principles and ways to impart knowledge to players than Lavin is used to.