It strikes me that not very many people here have had surgery on a joint. It isn't just the recovery period, or residual pain, but the joint stiffens up and often works just slightly differently than it did before. You essentially got a new hand a need to learn how to use it. Practice is one thing, but in-game there are so many different factors that if you are thinking about the joint at all you'll move differently than you did before, even if just the tiniest bit, and for a precision shooter a tiny bit is enough to throw off the shot. It will take him some time to get used to it, to make it do what he wants it to do, and maybe most importantly, to get back to shooting without thinking about it, which is when a shooter is at their best.
And frankly the only way to get through this is too shoot in-game, as much as possible. He's got several more 3-or-4-for-13-to-15 games to get back to form. But I believe he will get there.