Defense: C
The Tigers got off to a very good start on defense this year. There was a time where they were near the top of the league in defensive runs saved (DRS) and outs above average (OAA). Sadly, that time has passed.
The Tigers are currently tied for fourth in baseball with 49 errors as team (tied with the first-place Braves, funny enough). They are middle of the pack in both DRS and OAA now. They have come back down to earth on defense.
For how terrible Javier Baez has been with the bat this season, he's been one of the best defensive shortstops in the league. He's third in the league among SS with an OAA of 9. Weirdly enough, Zack Short has a higher DRA at shortstop than Baez this year (3 to 1, respectively).
Without Riley Greene, the Tigers outfield has not been good defensively. Matt Vierling isn't nearly as good in CF as he is in the corners. Kerry Carpenter is pretty bad out there. Andy Ibanez...let's just say he should never play the outfield.
So while the Tigers aren't horrible defensively, they aren't very good either. I think a C is fair.