The 28-year-old, who was the team's opening day starter in 2010, has made just seven relief appearances and pitched 16 big-league innings this season -- all since July 22.
He's gone 3-1 in 10 starts with two of the Rangers' minor-league affiliates and is being inserted into the rotation on a temporary basis to give an extra day's rest to Matt Harrison and others, according to Texas manager Ron Washington.
The Rangers, who lead the American League's West Division, have three scheduled days off between now and the end of the season
"When we started looking at it, all of that came into play," Washington said. "We're trying to keep our focus on today, not what's going to happen two and three weeks from now."
Feldman, who's allowed seven runs on 14 hits in 16 innings in the majors, tossed an inning in the Rangers' 7-3 loss at Toronto on July 31.
He's 3-1 against them in eight career appearances with a 1.73 earned run average.
Feldman is opposed by second-year righty Jeremy Hellickson, who'll face the Rangers for the first time in his 24th start of the season.
The 24-year-old was a 2-0 loser against Detroit on Aug. 25 after allowing two runs on six hits in seven innings.
He'd gotten win No. 11 of the season and No. 15 of his career a start earlier, when he went eight scoreless innings in an 8-0 defeat of Seattle.
The Rays are 5-5 in his last 10 starts.
Rays starter Wade Davis (8-8) allowed six runs in 4 1/3 innings -- his second- shortest outing of the season -- to take the loss.
Johnny Damon had two home runs, but they weren't enough to lift Tampa Bay to its first four-game sweep of Toronto.
On Sunday in Texas, Josh Hamilton crushed a two-run homer and hit a go-ahead RBI single during a four-run seventh inning to help the Rangers extend their lead in the West with a 9-5 victory over the Los Angeles Angels of Anaheim.
Elvis Andrus joined Hamilton with three hits and knocked in two runs for Texas, which holds a three-game advantage in the division after taking two of three from its closest competitor.
The Rangers won two of three when they visited the Rays from May 30-June 1.