When Garrett Whitlock was coming up through the minor leagues with the Yankees, they planned on him being a starting pitcher. When the Red Sox selected him in the Rule 5 Draft, they planned on him being a starting pitcher. The circumstances around his acquisition accelerated his timeline and he was used out of the bullpen as a result. While he embraced the role and performed admirably, he was still meant to be a starting pitcher.
Whitlock is 6’5”, 220 pounds. He has a solid fastball, a good changeup, and a good slider. Without knowing anything else, that screams rotation piece to me.