I don't see Lempa as a critical loss. He's always been well liked by players and his familiarity and ties to BC were always nice, but he's not some ace recruiter or great strategist. He also seems to consistently have weird relations with his BC bosses. TOB did not offer him a job at NC State and Spaz and Gene both wanted him out when Jags took over. His relationship with Don Brown is what brought him back to the Heights. It doesn't surprise me that with Brown gone, he's not staying with Addazio.
The only thing that Lempa might impact is the timing of the other Defensive replacements. Addazio might speed up the process a bit so that he's not short-staffed when the quiet period ends.