Once Tenuta was added to the mix the defense got substantially worse.
Brown's defenses were actually pretty good but ND's lack of depth usually doomed them and they would wear down in the 4th quarter (part of that was on Mendoza too).
The defense in 2009 under Tenuta is what got Weis fired. That team should have won 10 games easily but the defense blew it too many times after the offense had the game won - Michigan, UConn, Stanford, and Pitt all come to mind.