Also Martin was behind only 4 regular Jays in wRC+ this year, and Carrera was the only one who isn't a particularly good fielder.
In OBP Martin was one place better, with a much better OBP than Morales, or Pearce (a likely DH candidate if Morales wasn't available).
It's not a stretch to assume that if Martin was only catching 3 out of 5 games, that he'd DH about once a week if Morales wasn't tying up the DH spot.