What makes you believe that he's not the best SP the Jays have by a wide margin?
In 2014 he had 130 innings, in 2015 he had 27 innings.
Both of them are very similar in terms of results, at around xFIP- of 83. That means he's 17% better than the league average.
If you look at both 2014 and 2015, the only other starter that comes close to that performance is Price with xFIP- of 71, so 29% better than league average. But even that is a very small sample size. Price career numbers are closer to xFIP- of 80, so not far away from Stroman at a fraction of the cost.