He hasn’t actually started a rehab assignment, right? They can make a rehab assignment go 20 days, so unless it starts today or tomorrow, they don’t have to make a decision on him before season’s end.
Additionally, St. Paul only has two weeks left unless they make the playoffs.
The likelihood of him improving the team after this much time off is pretty slim. If they deem him ready, I’m guessing they send him to St. Paul for however long they play, then send him to the complex to get additional at bats, but the only way he gets added to the Twins roster is because of multiple injuries.