Bailey felt tightness in his arm when he threw off the mound prior to Friday's game vs. the Braves.
"Getting back on the mound, you get that kind of Spring Training tightness," Bailey said. "Nothing urgent. Just kind of, I don't know, nothing I've ever had before, or any pitcher has ever experienced. For us, before we take that next step, maybe throw another bullpen [session] or two and then we'll go from there."
The right-hander had been ahead of the curve in his rehab before Friday's setback.
"It didn't go perfectly," said manager Bobby Valentine. "We're going to let him rest a little before he gets back on the mound. His arm didn't feel great throwing. We were accelerating his progress a little and we decided to slow it down."
So how does the thumb feel?
"Thumb's feeling great. That rehab went really well," said Bailey, who has not pitched for Boston this year. "Now it's about getting the arm in shape as well."
The next time Bailey throws it will be from flat ground.
"I need to just be confident in letting it loose before I go face hitters or anything," he said. "Nothing urgent. It just takes some more time. We've got a lot of season left. I've got to be able to help the team, healthy and confident. That's what I've got to do."