"He passed every test that was given to him this spring," Cherington said of Sizemore. "Soon, he will have more tests to pass. In the time we had in spring training, the performance, combined with that, we just felt like that the best team we could put out there Monday has him on it.
"I think we felt all along that definitely for the Red Sox, over the course of this season, is to have both Grady and Jackie in the organization and at some point on the team. There were questions as to how to do that.
"Grady has done everything he could have done during spring training. He has been able to perform while passing the physical tests. I'm excited for him. He has worked his tail off. For a guy who has been unable to do what he always has done, it's a lot of fun watching him out there."
Sizemore hit .333(13-for-39) this spring training and Bradley Jr. hit just .158 (9-for-57) with a team-high 17 strikeouts.
Cherington said that also factored into the decision.
"I think it's one thing to be on the roster and be an active major league player," Cherington said of Bradley. "It's another thing entirely to be a guy that the team can count on to play a lot and to play a meaningful role. That's probably what we learned this spring. This is not someone that we envision as a bench player. This is someone we envision as a regular contributor who can play a lot. That's probably what we needed to learn in spring training.
"Jackie has had two short moments in time in his career when he hasn't performed. One of them was last year during the season and one was this spring. We obviously have seen a lot more moment from him in which he has performed. We're very confident he will do that. He wants to play. It was a difficult conversation, but he handled it very well and understands."