DETROIT -- Brian Johnson was cruising against the Tigers on Saturday. He had allowed two unearned runs in 5 innings. He had thrown 77 pitches (53 strikes).

So why didn't Alex Cora send him back out for the sixth inning? Cora instead replaced him with reliever Tyler Thornburg who gave up three runs, including a two-run homer to Jose Iglesias. The Red Sox lost 5-0.

Johnson still isn't completely stretched out. He has spent most of this season as a reliever. He returned to the starting rotation June 28. But he had started only one game in the previous 17 days entering Saturday. He spent 10 days on the DL with hip inflammation and also had time down because of the All-Star Break.

"I think 5 innings right now is good for him, 80 pitches," Cora said. "There was a high-leverage inning in the game and we felt that was good enough for him. He gave us a chance. Hopefully for the next one we go six. But that was a good job by BJ."

Johnson gave up five hits, didn't walk anybody and struck out five.

"I feel like after this I should be up to 100 pitches and six or seven innings," Johnson said. "Obviously saying that, I've got to talk to (pitching coach) Dana (LeVangie) and Alex about it. But that's what I would assume."

Could he have pitched another inning Saturday if Cora asked him?

"I think they wanted to go to Thorny there but yeah, I could have gone one more," Johnson said. "I felt good enough to go another one."

Johnson's ERA dropped from 4.20 to 3.81. He has a 2.22 ERA (24 1/3 innings, six earned runs) in five starts.

His fastball averaged only 88 mph and topped out at 91 mph Saturday. But he mixed in 16 sliders and 25 curveballs.

"BJ can pitch," Cora said. "He does a good job with what he has. Going up in the zone. Throwing the breaking ball. Expanding down-and-in against righties. It might be a comfortable at-bat but it actually is not. Probably what you see is not what you get as far as the pitch up in the zone. You set your sights up and that pitch is higher than what you think it is. We're very happy with the way he's throwing the ball. We're comfortable with him. We're going to keep rolling with him."

The southpaw posted a 9.00 ERA (13 earned runs, 13 innings) in his first 10 relief outings. He record a 2.12 ERA (17 innings, four earned runs) over his final 11 relief outings before returning to the starting rotation.

"Looking back, I think going to the bullpen's actually made some of my offspeed pitches better," he said. "I feel like my slider's come a long way from last year and the year before. And I think going to the bullpen's helped it to get better. For me, you come out of the bullpen almost like throwing two-strike pitches immediately. ... To go to the bullpen has allowed me to do that: to throw it harder and get more action on it."