I'd have Schwartzmann as favourite. Murray has more talent and ability, but his injuries mean he's not the player he was, and he can't sustain top-level performance for the duration of a tournament. His first round match was long and draining (3h45m), so I would pick Schwartzmann to win this one. But then my record in tennis betting is awful.
Seriously, I have no idea why they would put Murray as a favorite. He does still have his moments, but as you said his health is going to hold him back again and again as he can't have the same trust in his body that he used to have. It is not even something like a home game for Murray.
Especially if it is going to be a three sets game I would most likely bet on Schwartzmann. I wish for Murray to be able to perform at his best though. Always good to see people trying to fight their way back.
Yeah. I think I would probably just steer away from betting either way on a Murray match at the moment. Certainly Schwartzmann should be favourite, but Murray is the more talented player, and if he somehow has recovered and brings his best game and is able to hold it together, then he will win... so whilst Schwartzmann should win, there's just no guarantee at all, because there are such huge question marks over Murray. I think there are easier matches to bet on.