If you're playing chess that would be a dumb move. What's your end goal here? It would be to sell him for a premium. Dump him after those mistakes and that's a sure way to shave off 10-20% of his selling price. Regardless, when De Gea is good - he's really good. With potential 'finals' coming up for us, we're going to need him at really really good. He generally bounces back. However, by no means am I saying keep him long term. He must go. Someone posted a stat showing he is in his own stratosphere of mistakes leading to goals amongst keepers in the PL. That is enough to get him fired.
Champions League qualification - as
@Regulus Arcturus Black already said. The fallout, post-qualification would be an entirely different kettle of fish, but at least we're then in a position of strength not weakness.
As long as we qualify, I don't care who starts these two games, but if De Gea has one of his 'moments,' or two of them in the same game, or runs them concurrently, which can easily be identified as undeniable contributors to us not qualifying, heads will roll, and it won't just be bigwigs that accounts for. It cannot be played down - these are pivotal games for us, absolutely defining. The pressure on De Gea, should he get the nod, is going to be perhaps the biggest he has ever faced in his career to date, as, coming into these games after utterly calamitous errors is going to have the media on his back, as well as the oppo testing their luck in every which way they can.
Moyes will know De Gea's weaknesses well; bombardment of aerial balls he won't come for allied to his current shot-stopping crisis makes for a nervous 180 minutes unless our attack can keep the ball well away from the backline.
Ole is in a no-win situation in regard to either keeper letting him down, though. De Gea starts and blunders, and the discussion will be around why the hell he kept his place; Romero comes in and lets the side down, and it becomes: 'why didn't he keep faith with De Gea?' But ultimately, it's still his job, and our hopes on the line here. De Gea is a speck of dust by relative contrast.
In an ideal world, he fixes up and all of these discussions are redundant, but you're a braver man than me if you'd bet your house on things panning out that way.
As for selling him. That's not happening. Nobody is coming in for a player on that kind of wage with the faults he has in his game and the age he is. Yes, keepers mature and come into their own at their own pace, but a reflexive keeper like De Gea, even if he was still the same player as a few seasons back, won't have that many miles on his biological clock as history has shown reflexive keepers tend to lose it in their early 30's, unlike your all-rounders or generally masterful ones who have no problem being top class into their late 30's.