Plouffe originally bruised his thumb against the Royals on July 20, when he was jammed on an inside pitch from Royals right-hander Luke Hochevar.
He missed Minnesota's next five games as a result, but was slated to make his return on Friday. But Plouffe's thumb continued to bother him while taking batting practice before Friday's game. He was scratched from the lineup as a result with Jamey Carroll getting the start at third base.
"He didn't feel right in BP," Twins manager Ron Gardenhire said. "He swung earlier in the cage and was OK. But he got out there on the field and started swinging and felt it. No good."
Valencia, meanwhile, will be making his return to the Twins after being optioned to Triple-A on May 9 after hitting just .190 with one homer in 27 games.
In 69 games with the Red Wings, Valencia hit .250/.289/.399 with seven homers and 37 RBIs. Valencia, 27, hit .311 with seven homers in 85 games as a rookie in '10 before hitting .246 with 15 homers and 72 RBIs in 154 games last year.
"The numbers aren't great, but they're maybe not indicative," Twins general manager Terry Ryan said. "I'm sure he wasn't happy to be down there after spending two full years up here. But you have to get over that disappointment when you get a demotion."