The Red Sox went on to win the World Series.
"It's a motivating factor, period, in my career," said Thornton, who turns 38 in September and was activated by the Nationals on Wednesday. "I feel like I have a lot of baseball left in me. I feel like I have a lot of years ahead of me still. That was a very disappointing time in my career. … I don't need a whole lot of motivation in the offseason and during the season, but it was fun facing Boston this year."
That wasn't the only thing from 2013 that provided material for Thornton's proverbial bulletin board in the offseason. The left-hander spent the better part of eight seasons with the White Sox before being dealt to Boston at last season's Trade Deadline. During his time in Chicago, Thronton pitched alongside a number of talented right-handed relievers, including Nate Jones, Jesse Crain and Addison Reed.
"I didn't need to face righties," Thornton said.
That changed when he went to Boston. Thornton faced right-handed batters in 39 plate appearances during his abbreviated stint with the Red Sox, and they compiled a .353 batting average against him.
"Everyone said I was done facing righties," said Thornton, who compiled a 2.55 ERA in 46 appearances for the Yankees this season. "I took that as a little insult and focused on that and prepared myself to throw a full inning every day."
Thornton responded by performing better against righties than lefties in New York. In 65 games with the Yankees, right-handed batters hit 243 against him. Meanwhile, lefties hit .250.
Even so, manager Matt Williams said Thornton provides the Nationals bullpen, which has been scuffling of late after an impressive first half of the season, with the option to match up with left-handed hitters on a day-to-day basis. The team now carries three lefty relievers in Thornton, Ross Detwiler and Jerry Blevins.
"We've got some really good left-handed hitters within the division we can match up against," Williams said. "That's good. It's good to be able to do that."
Thornton made his Nationals debut on Wednesday night, getting two outs in the eighth inning while allowing an inherited runner to score.