Replacing the right-hander in mid-May as he recovers from a sprained right elbow, Feldman has lost all four of his starts and combined to give up 19 runs on 17 hits.
He'll try to change his luck Saturday, when the Rangers square off against the Giants at AT&T Park.
"It's tough to sit around and think about it," Feldman said. "I want to get that bad taste out of my mouth and get back to what I do best."
In his last outing Monday, the right-hander gave up eight runs on seven hits, lasting just 1 2/3 innings against the A's and increasing his ERA to 7.01.
"The main thing is just getting guys out," Feldman said. "Make an adjustment because I got hit around pretty good. Try to keep my confidence up and hopefully I can get back on track."
Ryan Vogelsong will be on the hill for San Francisco, coming off one of his best starts of the season.
Picking up his fourth win of the season Monday against the Cubs, the right-hander surrendered just two runs on eight hits. Winning his last four decisions, Vogelsong took his last loss May 3 against the Marlins.
"I felt like that was definitely the best all-around stuff I had," Vogelsong said after his last start. "It's kind of surprising I let up that many hits, but that was probably the best I've felt stuff-wise this season."
Rangers: Darvish to get extra rest
With an off-day Monday, the Rangers said Friday that they would use it to give Yu Darvish extra rest.
Instead of pitching Wednesday against the D-backs, Darvish will be pushed back and start against the Astros on Friday. Darvish is 0-2 with a 6.17 ERA in his last two starts after allowing six runs in 5 1/3 innings of a 7-1 loss to the Athletics on Thursday.
"He's fine, he just had a bad outing," manager Ron Washington said. "A couple of extra days won't hurt him. It will probably do him some good. He just had some trouble with his mechanics and couldn't make the adjustment. Usually he does. He's fine physically."
Giants: Melky sits out opener
Outfielder Melky Cabrera was out of the Giants' lineup for Friday's game against the Rangers with a sore right hamstring. Manager Bruce Bochy said Cabrera is day to day.
Nate Schierholtz started in right field, with Gregor Blanco sliding over to Cabrera's spot at left field, and center fielder Angel Pagan batted No. 3 in the lineup.
Cabrera injured his hamstring during Thursday's 8-3 win against the Padres while stealing second base in the seventh inning. He left the game an inning later.
"We took him out just to be cautious there," Bochy said Friday. "It's still a little bit sore so Nate gets the start today. We'll check on him tomorrow and hopefully he'll be good to go tomorrow.
Since being activated from the disabled list after an elbow injury, Giants infielder Ryan Theriot has batted .380 (19-for-50).
The Rangers are not yet ready to say when Roy Oswalt will be ready to pitch in the Major Leagues. The only plan right for now is to have Oswalt make his third start for Triple-A Round Rock on Tuesday at Albuquerque.