I was speculating something similar in another thread. But I'm also thinking that if he does need TJS, the Angels will take it easy with him afterward, which may also mean that they'd delay TJS until after the season - or at least until after they're eliminated - because they'll probably go conservative on treatment for a couple months to see how he responds, and it probably doesn't make much difference if he gets cut in August vs. October. Either way he'll miss all of 2019 and should be ready in 2020.
So if he needs TJS (and that is still an if), then we might see something like this:
June - July: Shutdown completely while stem cell injections commence, slowly returns to hitting
August - September: Hitter only
October: Tommy John Surgery
October - March: Shutdown
March, 2019: Resumes baseball activities as hitter; main DH during season
March, 2020: Returns as pitcher/hitter