Thames has become an everyday player during 44 games with the club this season. He got the start vs. Rangers southpaw C.J. Wilson on Sunday in part because of his overall production against lefties.
The 24-year-old Thames is batting just .226 (7-for-31) against southpaws, but he does have two home runs and six RBIs.
"I think the numbers bear it out," manager John Farrell said of Thames starting. "He has put up good at-bats against left-handers. He has stayed in on some balls, he has driven some balls the other way.
"The fact that he has been in a good run, a productive run, and [I] just feel like right now he's probably a little bit more equipped to handle a tough lefty and we're in the process of finding out about him as well."
The same every day role can't be said for left fielder Travis Snider, who has not received a similar opportunity. Snider wasn't in the starting lineup Sunday, but Farrell didn't want to close the door on starting against lefties down the road.
Snider is batting .125 (5-for-40) with one home run and six RBIs against left-handers this year. During his four seasons in the Majors, the Washington native has posted a .216 average in 153 at-bats against southpaws.
Farrell said the decision to sit Snider on Sunday made sense because he wanted to find a way to get outfielder Rajai Davis into the starting lineup and also provide J.P. Arencibia with a chance to serve as the designated hitter.
"I don't think that we're at the point of saying Travis is solely a platoon guy," Farrell said.
"This isn't a matter of limiting Travis to left-handers only."