The Red Sox signed Cuban free agent Rusney Castillo a few weeks ago, and squeezed every plate appearance they could out of the minor league postseason to get the rust off of him before bringing him to the bigs for the first time. Castillo debuted in the majors on Wednesday night while batting seventh and playing in center, and picked up his first big-league hit in his first game.
Castillo was probably going to be safe even if Neil Walker had fielded that grounder cleanly: Rusney is super fast, and the fact Walker had to dive and collect himself very likely created enough extra time for Castillo to make it to first with an infield single. Walker botched the post-dive action, though, so we didn't need to (or get to) see his arm vs. Castillo's wheels. You can see what I mean in the full video, though, which gives an alternate angle where you can see Castillo's progress on his way to first base against Walker's fielding.
Later on in the game, Castillo got the chance to show off his legs all the same by making a running grab in center on a Francisco Liriano fly ball.
I know we all miss Jackie Bradley Jr.'s defense, but between Mookie Betts and Castillo, the Sox will not be lacking for range in center in the future.