BRADENTON, Fla. — Red Sox pitcher Clay Buchholz struck out four and walked two in five innings in Boston’s 4-3 loss to the Pittsburgh Pirates Monday.

Neil Walker’s leadoff homer in the second ended Clay Buchholz’s 91/3-inning scoreless streak. It was the only hit off Buchholz.

“(Buchholz has) gone through a pretty drastic transformation from the way he pitched in the minors and in his rookie year to now,” Red Sox Manager John Farrell said. “The last couple outings, when he puts the ball on the ground, he’s at his best. That’s when he’s most relaxed.”

After recovering from a stress fracture in his back in 2011, Buchholz began last season by going 4-2 with a 7.19 ERA in his first 10 starts. He was 7-6 with a 3.45 ERA the rest of the way.

After watching video of Buchholz’s early outings, Farrell said he saw a lot of pitches up in the zone.

“Maybe he was subconsciously protecting (his back) or just couldn’t get full extension to drive the ball downhill,” Farrell said.

“But after his seventh or eighth start, he ran off good numbers, like he was his old self. That’s what we’re seeing now.”

Pittsburgh starter James McDonald labored in the first inning, then recovered.

“I got locked back in, kept my composure and finished strong,” McDonald said. “I didn’t let the game get away from me. I felt confident – every pitch, every moment out there.”

Ryan Lavarnway hit a two-run single in the first. McDonald then retired 11 of 14 batters. He gave up four hits and three walks in five innings and struck out six.

This was the second game in a row that McDonald overcame a rocky first inning.

McDonald has a 4.19 ERA in five games with four strikeouts in 19 1/3 innings.

Pittsburgh’s Gaby Sanchez hit a two-run homer in the seventh off Chris Carpenter, and Michael McKenry hit a solo shot off Oscar Villarreal in the eighth.

NOTES: Red Sox DH David Ortiz’s scheduled batting practice was canceled due to continued pain in both heels. “He’s not ready for that next phase,” Farrell said.… If SS Stephen Drew continues to be bothered by post-concussion symptoms through Tuesday, the Red Sox would send him to be examined by Dr. Michael Collins, a specialist in Pittsburgh. “When he goes through exertion tests there’s no increase in the symptoms, so we take that as a positive,” Farrell said.… Red Sox LHP Franklin Morales (bulging disk) threw from 60 feet on Monday. “If we can repeat that a couple of days, we’ll have more insight on what plan to follow,” Farrell said.