What surprises, is that they don't have a third driver for this? Anyone could have an accident, or simply get sick any time.
N.
They have two test drivers, but they are rather inexperienced and at least for Ferrari, less a choice than grabbing DEEEEP into the pocket to bring Schumacher back (You can be sure, Schumacher will not only return for just the fun of it, though I am sure he will enjoy racing again)
Also, the psychological effect is extreme. 2 days ago, nobody would have bet on one of the Ferrari pilots becoming champions this season, today people believe that Schumacher will win the season automatically.
I doubt that Schumacher is really able to fix all troubles with the car and the team, but I am also sure his skills would make the season more exciting. Schumacher was always able to squeeze the optimum out of the worst Ferrari cars and I am sure even if he his 20% away from his optimal physical shape and reactions, he could do a lot...
And it is also not the worst situation for Massa, even if the public will likely make it hard for him to return into the season and make Schumacher retire again: Who else can claim of having THE formula one legend as deputy?