Just not being good isn't cause for an IL stint.
I'm not suggesting it's hard to find some reason but you can't just say "He can't get minor league guys out" and IL a guy.
I think there may have been some truth to that back in the day, but in today's game the Umps get post game feedback on every call they make. They (with rare exceptions) are not going to deviate from what they think are balls and strikes.