After scanning the medical paper @Eris linked, it says 5% of all surgeries for injured baseball players are knee surgeries. That is all knee surgeries not just the ACL. It does state that knee injuries are more common for outfielders, but I couldn't find any stats on that. The paper categorized knee injuries as relatively rare in baseball it being a non-contact sport.
After mulling this over, it doesn't seem to matter whether Lewis plays infield or outfield. I do think if he is more comfortable in the infield, then I would leave him there for now as he makes his way back post surgery. As suggested above, later this year he should start getting practice reps in the outfield for future positional flexibility. Then later this year as things play out with Buxton, Celestino, MAT and Martin, you chart your coarse forward.
Any athlete in any sport has to learn how to protect themselves during play. Some are better than others. Lewis still can get there and be healthy going forward. The big question is, is it too late for Buxton? Has he already done too much damage earlier in his career?