In the games he was functional, yes he quite clearly did. It's going to relieve a lot of pressure from not only the perimeter defenders, but there'll be less shyt the bigs have to contest in the paint/at the rim.
How can they stink as a collective when they've been a top-10 defense for 90% of the season, without their best perimeter defender? Any team in the league would be going through similar struggles if they too were missing arguably their best defender.
Of course this team lives and dies with Lowry/DeRozan, that doesn't even need to be said. Point is, DMC is going to cover a lot of holes in the defense and allow the team to develop more consistency on that end. If you're of the belief that Demar is a net negative defender and causes the team issues on that end, how won't the return of a healthy DeMarre help fix most of those issues?
DeMarre will have an effect on the whole team, because everyone will have less defensive duties to deal with - therefore having a domino effect on how the team functions on both sides of the ball.