Ryan Lavarnway's two-run single off James McDonald in the first gave Boston a quick 2-0 lead.
Buchholz gave up his first run of the spring, but he was extremely sharp for a fourth consecutive start, limiting the Pirates to a second-inning homer by Neil Walker. In 13 1/3 innings, the righty has allowed seven hits and a run.
For Walker, who was hampered by injuries the last six weeks of the 2012 season, the homer was his first since Aug. 12.
For his second straight start, McDonald endured a shaky first inning and then settled into dominance. On Wednesday, he allowed three runs in the first to Toronto and then retired 14 of the last 17 batters he faced. In Monday's game, he gave up two runs on three first-inning hits and then held the Red Sox to one other hit over his final 4 1/3 frames.
The right-hander also notched six strikeouts and issued three free passes.
Jason Grilli, the Pirates' new closer who had been away participating in the World Baseball Classic with Italy, made his first Grapefruit League appearance since March 3. Grilli came on in the sixth and retired the side in order.
Up next: The Red Sox return home Tuesday against the Orioles in a 1:35 p.m. ET game at JetBlue Park. Right-hander Ryan Dempster is the scheduled starter against Baltimore southpaw Brian Matusz. Boston righties Terry Doyle, Junichi Tazawa and Daniel Bard are also slated to see action.
Tom Singer is a reporter for MLB.com and writes an MLBlog Change for a Nickel. He can also be found on Twitter @Tom_Singer. This story was not subject to the approval of Major League Baseball or its clubs.