I'm not sure that would be a good idea. The one thing guaranteed to make it hard for a keeper to make a good start in a new side is to not commit to him straight away as #1. Drop him after a good game and he can't build momentum. Drop him after a bad one and it piles too much pressure on mistakes. We made a success of De Gea by sticking with him despite those early mistakes and the hysterical media pressure at the time.
Plus after everything De Gea has done for us - not just his brilliance, but his consummate professionalism amidst the almost perennial transfer uncertainty - we owe it to him to make it a clean break when we do decide to move on. When we're ready, we can sell him to Madrid and send him off with best wishes and no hard feelings, not muck him around giving him half game time for a year. After all, we have the best, most reliable #2 in the league in Romero, so an excellent insurance policy against Henderson not making the grade.