Why not he was willing to do it earlier in the year. I'm not saying Osuna isn't going to help us in the playoffs because he probably will but defined roles and rest goes out the window. It shifts to the hot hands and doing whatever it takes to win.
This isn't the try league this is get the job done league. In tight situations you go to your best reliever. That isn't Roberto Osuna. Closers don't exist in the playoffs. Especially ones with 5 minutes of success.