I'm not 100% sure but from my understanding if the cubs DFA him he has the choice whether to accept the assignment or become a free agent. Typically players don't accept it because they got their money anyways and maybe they can latch on with another team as a MLB player. But my thought here was that if indeed he wants to stay in chicago he may realize that at the end of spring training he may not have a role on the 2019 cubs. However, injuries happen and theoretically he might have a role in 2020 were he to fix his issues.
Long story short, it would be an unusual situation but he has said he wants to be a starter for a team that doesn't have an obvious opening for one. He's also said he doesn't want to be traded to start some where else. So, my thought was this gives him a way of staying with a team he wants to play for while giving him an opportunity to fix his issues and a potential opening should someone get hurt. I mean if any starter were to go down him and i guess monty would likely be your two highest priority guys though the cubs lack lefty depth in the bullpen. So, if he's pitching even some what reasonably a la 2017 he'd probably be the guy.