He was also just plain off earlier. Like the long pass to Gabriel in one of the first vcouple of games where it seemed he seriously underestimated Gabriel's speed. A lot of it seems to be that he just got his wide receivers three weeks ago. Look at who he hit on passes. He wasn't missing on passes to Troy Brown...but was missing pretty much all the rest of his WRs. By mid-season, Gabriel will be a serious threat.
I think Gabriel is going to be a decent pick up for them. He is certainly tough enough to make catches and get an extra yard or two when needed.
Part of the problem with new receivers is that they break off routes or run the wrong routes. They don't get to spots that the old and more familiar guys used to. However, with a good ground game you can cover the passing game until they catch up. Caldwell and Gabriel have played in the NFL before so it's not like they have no clue on what a WR is supposed to do.