Rielly can be a #1 if you have say Hague-Whitecloud and Martinez-McNabb behind him, and his partner is Schenn. Or you have a Rielly and a very comparable player as co-#1's, each on their own pairing. Anything less than that and you're likely not gunna be winning a cup. No Muzzin equivalent will change that, unless you're solid in net and have buy in from the forwards (which obviously the Leafs have neither).
Let's say the D magically becomes Rielly, prime Muzzin equivalent, Brodie, McCabe, Liljegren, and whoever you wanna call the #6. That's a low end #1, a mid range #2-4, a great #5, a below average #6, all totaling a painfully average defence group.