I don't know how to calculate ERA+ but Johnson's WHIP as a starter was 1.451 (4.15 ERA, 7.9 K/9).FWIW, only 60.2 of Brian Johnson's 99 innings pitched in 2018 were as a starter. I can't figure out how to pull those out to get his ERA+ or whip info, though. Eovaldi also had 50 innings of starts for the Sox with a total ERA+ of 133 for the Sox (he also had one relief appearance)
Eovaldi's one non-start that season was following Sale's return from the IL. Effectively, he was the long man after Sale was the opener. Hardly worth parsing those 3.2 innings at all, especially considering that was a longer outing than his final start of the year...a planned two inning outing on the next to last day of the season.
To my point about the fifth spot generally being a collection of pitchers, Johnson and Eovaldi combined for 24 starts (111 innings) with a 3.73 ERA, a 1.37 WHIP, and 7.9 K/9. Unsurprising that a 108 win team would have a "fifth" starter with those numbers.