His biggest problem as a starter has been giving up a ton of homers (seems to be a problem with all Sox starters, actually). His numbers as a reliever seem to be a little fluky, that .235 BABIP seems unsustainable.
SP: 1.7 HR, 2.9 BB, 8.6 K, 844 OPS, .329 BABIP
RP: 1.2 HR, 3.1 BB, 9.6 K, 592 OPS, .235 BABIP
He does seem to have a times through the order split. As a starter, 763 ops first time, 859 second time, 1032 third time+. As a reliever, he has been better against batters the second time he’s faced them but it’s a fairly small sample.
Ultimately though, I think I agree that the SP/RP splits probably don’t mean much and might as well give him the rotation slot; more theoretical upside than the other options.