I think JD has it right. Castro is essentially the 5th OF and 6th IF behind the starters and Margot/Farmer, respectively. His xwOBA was about 20 points lower than his wOBA last year, so even though he hit the ball harder and at a better LA than his career averages, he's probably due to hit a bit worse, although it is encouraging that his BB rate has trended up given what he can do on the bases.
Castro's versatility is helpful to this roster. Buxton is going to need days off for minor injuries and maintenance even if everything goes wonderfully. Margot is Buxton's top reserve, but Wallner needs a platoon partner against tough (or maybe all?) lefties. It's not realistic to think that Buxton will always be healthy on days where the team faces tough LHP, so we really need two reserve OFs who can hit LHP. Having someone in that role who can also provide IF depth to give Lewis and Correa rest days will be helpful. Of course, the depth is not very useful if he reverts to a low .600s OPS, so hopefully he maintains some of the offensive gains.
