Buchholz has been on the disabled list since June 17 with a lower back strain.
"We want Buch to see one more specialist on Friday or Monday-ish before we completely turn him loose," Red Sox manager Terry Francona said. "He's starting to get aggressive and with everything that has gone on [with the back], we just want to make sure this kid is OK."
Francona did not seem overly concerned with Buchholz's skipping the bullpen session because the right-hander responded well from the one he threw Monday. Francona said Buchholz will not throw another session until he is examined by the specialist.
"It's a lot to ask moving forward," Francona said. "If he's going to start pitching in games we just want to make sure we're doing the right thing."
Buchholz tossed a successful 30-pitch session Monday at Fenway. He experienced "general soreness" on Tuesday, which was expected.
His arm strength has appeared to be where it needs to be during his rehab.
"He looked [Monday] like he was in midseason form," Francona said. "That's what was very encouraging to us. At the same time, we have an obligation to do the right thing."
Joe McDonald covers the Red Sox for ESPNBoston.com.