Well, I think it will probably take Altherr a full year to be truly recovered (physically and mentally). I believe Texeira, who was admittedly much older, thought it took 18 months. But I don't think he's gonna get enough playing time to regain his timing/power and show enough to unseat two high-paid vets (and especially not another righthander). Of course if Kendrick is as bad as he was last year there's an opening, and I guess you are assuming they will be hoping to trade Saunders. But it's just as likely Roman Quinn will replace him. And if the team is exceeding expectations by hanging around .500, I don't know that they will trade Saunders, given his (relative) youth and the option. If he is playing well and the team is winning they won't want to mess with that, and they could also exercise his option and still trade him in the off-season.
In the best possible world, Phillies 2018 OF is Williams/Herrera/Quinn (with Cozens knocking on the door) and Altherr has either been traded or is a 4th OF. I suppose it is also possible Quinn is best-suited to be a 4th OF, if he's not durable.