Bumping this thread after no posting in ten days. The thought in the OP was that Castro might develop into a "unassigned regular", which has happened so far this year, mostly due to the injuries of Buxton, Correa and Lewis. IMHO, the overall results have been uneven. Willi has a 60% stolen base success rate (well below average) and hasn't contributed that much in the base running area. He struggled as the primary center fielder and was acceptable as the primary shortstop when Correa was injured. BBRef has Willi at -5 runs on defense and 0 for base running. Last year, Willi provided +4 runs from base running and +5 from fielding.
Castro leads the 2024 Twins in plate appearances and I believe he is a plus defender in left field and third base, but he has been stretched into playing more center field and shortstop. The offense has held up and looking ahead, playing Castro primarily in positions where he's a better defender, he's a very valuable member of the team.