If anything starting Gausman over Manoah would have been more of a question mark.
Years on the Jays: M=2 ; G=1
Cy Young finish: M=3rd (87 points) ; G=9th (1 Point)
FWAR: M=4.1 ; G=5.7
BWAR: M=5.9 ; G=3.0
Wins, ERA, Innings all things the majority of fans look at all favour Manoah.
Also helps form a bond with Manoah, might make him more likely to stay. If he sees himself as the #1 it might make it just slightly harder for him to leave. The Jays are grooming him.