To clear room, Boston optioned relief pitcher Heath Hembree to Triple-A Pawtucket and placed outfielder Daniel Nava on the 15-day disabled list with a left thumb strain. The Red Sox also designated utility player Jeff Bianchi for assignment.
Manager John Farrell said the team brought up Ross to add another arm to the bullpen as the Red Sox temporarily move to a six-man starting rotation.
The Red Sox acquired Peguero from the Rangers on Wednesday for cash considerations after Texas designated him for assignment May 20. Peguero, an outfielder, hit .186 with four home runs in 30 games for the Rangers this year.
"I'm surprised to be here and glad to be here," Peguero said. "I was worried what was going to happen. I'm grateful for the opportunity."
• The Red Sox's Brock Holt was born in Fort Worth, Texas, and currently has a home in Stephenville. Holt said he expects to have a large amount of family attending the series.
• The Rangers have won three consecutive series, including taking two of three last week against the Red Sox, but Texas is 0-5-1 in home series this year.
• After hitting two home runs Wednesday against the Twins, Dustin Pedroia has seven this season, which matches his total from all of last year.
Cody Stavenhagen is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.