Manager Ozzie Guillen announced after Saturday's game against the Oakland Athletics, that Thornton would be his man for the ninth inning of games when the White Sox were in the lead. Thornton did not pitch Saturday.
"I think Matt Thornton earned it," Guillen said. "I think we have a lot of confidence [in him]."
Chris Sale was the last reliever in the running for the role, but has struggled with a 6.48 ERA over 8 1/3 innings. Thornton has a 3.00 ERA over six innings this spring.
"Now in my opinion, I think Sale, we [would] have to put a lot of things on his shoulders, not just because this kid pitched well last season, but we would put a lot of pressure on him to be the closer," Guillen said.
Guillen did give himself some wiggle room, though, to make a change later in the season.
"I talked to Thornton and I said he would get more than the chance to be the closer," Guillen said. "Obviously when you're closing, I talked to him about it, a good percent of the time, he will be the guy."
As a setup man last season, when he was named to his first All-Star team, Thornton still managed to save a career-high eight games. He set other career bests by holding opponents to a .191 batting average, while posting a 2.67 ERA. His 81 strikeouts led all American League relievers.
Thornton's extension, which will kick in at the start of the 2012 season, was for two years and $12 million with an option for a third year.
Doug Padilla covers the White Sox for ESPNChicago.com and ESPN 1000.