The right-hander tossed 30 pitches and used each pitch in his repertoire. Red Sox utility man Conor Jackson stood in the box while Buchholz threw at 75 to 80 percent capacity.
"I felt good," Buchholz said. "There's no issue with my back, and I felt fine. I'm pretty optimistic about things."
If Buchholz feels good again on Sunday, it's possible he could throw a simulated game either Monday or Tuesday, according to Red Sox manger Terry Francona. The next step for Buchholz will also depend on field availability because the Red Sox host a doubleheader against the Orioles on Monday, so that could mean Buchholz will need to wait until Tuesday to throw a simulated game.
If he continues to progress, he's scheduled to throw a second simulated game on Thursday, and it's possible he could see some game action in the final series of the season at Baltimore.
"It's possible. That's the goal only if it's supposed to be," Francona said. "We're cautious, but we haven't pulled the plug on the season. Clay's done a terrific job and we'll see."
If and when Buchholz returns, it's likely the starter will be converted into a reliever for the postseason.
"I don't know if that's the role I want to be in," he said. "I want to start, but I don't know if there will be enough time to build up to that. If the team thinks I can help in a different role, I'll do it. If not, I don't want to make the team worse."
His goal at the start of this process has been to be able to return and help his teammates in the postseason.
Also, fellow pitcher Erik Bedard (lat and knee) played catch on Friday and felt good, so he also threw a side session on Saturday morning. It's possible he could be ready to start the matinee game of the doubleheader on Monday.
"I don't know, we'll see," Francona said. "We've got some moving parts right now. We want to see how he comes through the side, and then we want to gauge where he is, and then we can make some decisions."
Francona did announce that veteran right-hander John Lackey will start the nightcap against Baltimore on Monday.
Meanwhile, Red Sox reliever Bobby Jenks, who was recently diagnosed with pulmonary embolism, which occurs when the main artery of lung or one of its braches becomes blocked, said he will have back surgery once the clots go away.
"That whole thing is I've got two little growths that are coming off my spine, like two little hooks that have caused damage to my ligament just so much over the years that it's actually calcified the ligament and is now causing nerve damage going into the left side, like underneath my scapula," he said. "That's why we thought for so long that it was muscle but it was actually all the nerves that are being locked down from the calcified tendon."
The pitcher added that it's a simple procedure, and he's optimistic he'll be ready for spring training next season.
Jenks has remained with the team but hasn't been seen around the clubhouse until he met with the media Saturday morning.
"Very scary because with something like that it can be very serious," Jenks said of the embolism. "I'm in great hands. All the doctors have been wonderful, been taking great care of me. It's been a little bit of a relief as far as mentally-wise knowing that I'm in such good hands here."
Joe McDonald covers the Red Sox for ESPNBoston.com.