As a baseball player that pitches and plays most positions in the field, including catcher, I can relate a bit. Most players are gamers and want to be involved as much as possible. They love the game and the bench is the last place they'd want to be, as long as they're qualified to remain in the game.

As for this unique player, I think playing the field in the NL would be a horrible idea because it will prevent him from getting full rest between starts. Even as a professional, playing a full game, after a long starting pitching performance and potential traveling across time zones, is tough on the body to recover in time. It could also increase injury, not just from being in the field, but from the extra stress on their arm making laser throws from the warning track at full velocity to try to gun down players. After 100 pitches in prior days, no matter how conditioned you are, that's not a wise thing to do. When I pitch, I'm fine to stay in the game, or come in from the field in relief, but once the games(s) are over for the day, my arm and legs are at 70% at best for a few days. In a MLB setting, not being at 90+% defensive performance is going to hurt the team, as much as a potential (developing/unproven vs MLB pitching) bat he may have. It may also wear down their pitching over the season too. It's a long season and his stats have been very limited to prove such durability. As he gets older, it'll get harder.

Sure, Shohei is young and wants to prove himself as the next Babe Ruth, but in reality, it could be a disaster if he tears something in his legs putting the brakes on a ball in the gap to get the throw off.

Just my two cents. For longevity and overall health, stick to the safer league, the AL -- and let the professional DH do their job. Maybe plug in as DH occasionally on matchups. If they get an interleague start as an AL pitcher, then they can show off their bat a few times a year for the fans, if they're not DH'ing much.