I think Murray has always been in that top 5-ish range, and we're just now seeing more of his ability because the line is better and the zone blocking is working out. Actually, I shouldn't say the OL, because the WRs and TEs and other backs are blocking, too. All around, we're executing the running plays better, not getting holding calls, etc.
I will say, though, you don't get on my top RB list averaging .75 fumbles/game. Keep it up, hold onto the ball, keep scoring rushing touchdowns from outside the 10, and then we're talking.
I do love how DM pass protects, though. That's one great reason to keep him on the field, even during his fumbling spell. He's really underrated on passing downs still, somehow.
All that said, I still don't pay him a deal that averages more than $4.5M/year. We can find another good back, or run it by committee if we have to. We need that money for making this defense a dominating unit at some point in the not-ridiculous future.