It is questionable if he can improve the teams motivation and form. His health was directly related to those things as he might not have been able to fire the team up due to his failing health. Ill give you that. Even then though, its a gamble. Just look at his recent track record for substitutions/lineups after his treatments and its more of the same, sub-par stuff.
Man, forget about his substitutions, lineups and motivational skills. These are rather minor issues. The biggest, more fundamental problem is the total tactical disarray which I'm observing on the pitch. Look at this:
Look how organized they are under del Bosque. They have a concept and you can clearly see it. Look at those quick one-touch lay-offs inbetween the lines. Those are some insanely sexy moves. Look also how compact they are. It looks like they always have 5 or 6 players very close to each other, condensing the pitch so they can immediately win the ball back while attacking in numbers. They are doing this with Villa, Pedro, Iniesta, Xavi, Busquets, Pique and Valdes on the pitch. It's basically Barca without Messi. And they don't need Messi to create chances against a very strong team (Ribery, Benzema, Matuidi, Cabaye, Pogba, Evra, Varane, Koscielny, Lloris, Valbuena ...).
And then compare it with Barca this season where the biggest attacking threat comes from Messi's individual skills and that's it. Then you have Dani Alves doing whatever he wants on the right side, pumping in one mindless cross after another into the box, Messi trotting around most of the time, Pedro and Alexis being non-factors because they are far too wide and look disconnected from the rest of their teammates, shitty transitioning, leaving huge spaces open and getting counterattacked in every single big game etc. etc. etc. It's just sad.