BOSTON — A few days ago, Chris Smith seemed a good candidate to be heading back to Pawtucket without having thrown a pitch in the big leagues.

Last night, though, the 27-year-old right-hander was reveling in his first win in the majors.

Smith was the beneficiary of the Red Sox’ four-run rally in the eighth that turned a 4-1 deficit into a 5-4 lead. Smith, who had thrown two scoreless innings, watched as Jonathan Papelbon nailed down the win for Boston and for Smith.

“This is really exciting stuff,” said Smith, who made his big-league debut on Saturday, a four-inning outing against St. Louis, in his second stint with Boston this season.

“To get a win, wow,” said Smith, in his seventh year as a professional. “To get a win as a reliever, they don’t come along too often. I’ve got the ball and the scorecard. I’ll just box them up.”

Smith, who remained with the Sox because Mike Timlin went on the disabled list, kept the game at 4-1 by blanking the Diamondbacks for two innings in relief of Justin Masterson. He had a scare when Chad Tracy’s long high drive to right was hauled in by J.D. Drew at the fence past the Pesky Pole in the seventh.

Smith then sat and watched the Sox’ offense go to work in the eighth. When Boston went ahead, Papelbon came in and Smith realized he had a chance for a win.

“It was like, ‘Wow, cool, he can take the reins,’ ” said Smith of Papelbon. “It was ‘Holy (mackerel), I might get the win. Then it happened. Damn, I’m the winning pitcher.’ Everything happens up here so fast. It’s going 100 mph. My head hurts. You can get a little dizzy up here.”

Around the bases

For the third straight game, the start of last night’s game was delayed by rain. … Jonathan Papelbon, who notched his 22nd save, fanned two hitters in the ninth. That marked the 17th time this season that the Sox’ closer has fanned at least two hitters in an outing, the most by a Red Sox pitcher through 80 games since Mark Clear did so in 1982. … Papelbon’s walk of Eric Byrnes with one out in the ninth was just the sixth walk he had issued this season. … The Red Sox now lead baseball with 12 wins in their final at-bat. … The Sox improved to 9-5 in interleague play this season. They need to win three of their final four interleague games — tonight against Arizona and three against Houston this weekend — to match their interleague win total of a year ago.