Mateo's role should be as a super util guy. Get him playing 2B and 3B and he's a decent bench guy a few times a week. If he gets his OPS+ near 100 all the better. He's not the starter on a team that expects to win 100 games though. Not nearly enough upside in that bat.
I'm a bit confused by your wording...do you think if that Mateo got his OPS+ near 100 that he still isn't good enough to be a starter even on a 100-win team? He'd probably be something like 4 WAR if he could hit like that. The Yankees and Astros have lesser players than that at SS and they'll likely both win 100 games this year.