Fergy is certainly the better pass-catcher and athlete, and he's certainly smart enough to learn quickly.
But let's face it - on the plays where the TE is (say, less than five per game) not targeted he's blocking (call it 60 per game) and must learn every blocking assgnment, and every opponent he must face on the edge and in the secondary.
That's 90 percent of the workload and where experience comes in - and we know that McVay is reluctant to play anybody who isn't "there" yet to the point of not making predictable mistakes.