I certainly have thought this was the case. His best season after the Sox stole him from the Yankees via Rule 5 was his first and entirely in the bullpen.
However, three factors have militated against that. The first is that he definitely has the repertoire for a starter with fastball, cut fastball, slider, and changeup. The second is that he really wants to be a starter. And the third is the Sox shortage of starters this year, last year, and in 2021, Whitlock's first season.
All the same discussion applies to Houck although he is less injury-prone than Whitlock and doesn't have the repertoire.
Both have been exclusive starters this season because of the Sox shortage of adequate starters. Paxton and Sale have both spent lots of time on the IL. Kluber was/is a bust. Crawford was a bust but is back in the rotation. Bello was called up out of need and is now flourishing.
Right now, without Houck and Whitlock--both of whom talksoxers insist should be in the bullpen and not the rotation--the Sox have three starters: Paxton, Bello, and Crawford.