I'm not worried yet. My take is for years we've all commented on how our WR's don't get "separation". As a unit they're not that fast, we don't have a Steve Smith or DeSean Jackson burner.
When Kurt took over there were multiple threads on here about how Kurt had to learn to trust the WR's, especially Fitz and Boldin. They weren't running free all the time like in his Ram heyday, but they were so good if he just gave them a chance, they'd make the play. Kurt eventually did figure this out and we saw some amazing plays where Kurt put the ball where only the WR had a chance to get it, and they did.
I suspect Matt is in that same process, but without Boldin, so he's still learning to trust. If you just wait for someone to be wide open, you're going to miss some chances in every game.
I still say Matt played pretty well in the opener, they had heat on him pretty consistently, but Matt for the most part made the right decisions and he clearly looked to have more velocity than when we last saw him. Remember this is a guy whose legend was built on having the guts to make that long throw against Notre Dame, to throw deep in the first series of his first NFL start(think it was KC). Matt didn't look at all scared against that vaunted Bears defense on MNF in the debacle game. But he has been sitting behind a HOF QB the last few years and he's got to get that swagger back.