ARLINGTON -- After staying on the injured list for nearly three months to begin the season, Rangers right-hander Max Scherzer will spend at least 15 more days there, as the club announced Friday that he was headed to the IL with right shoulder fatigue.

The Rangers recalled lefty Walter Pennington, who was recently acquired via trade from the Royals, from Triple-A Round Rock in Scherzer’s stead.

Scherzer, who turned 40 on July 27, endured a lengthy rehab process after offseason back surgery. But he has been a regular in the rotation since returning to the Rangers on June 23. He hadn’t missed a turn in eight starts, posting a 3.89 ERA in 39 1/3 innings with 38 strikeouts.

Manager Bruce Bochy said after Friday's 11-6 loss to the Red Sox that he expects the right-hander to return as soon as he is eligible to come off the IL, which would be Aug. 17.

“He actually thinks in a couple more days he could be ready, but we have some concern for him,” Bochy said. “We want to take care of him and have him for the rest of the year. That's why we're giving him a break right now.”

Arm fatigue has become an issue of late, though, shortening two of Scherzer’s past three starts. In his most recent outing, on July 30 in St. Louis, Scherzer departed after 68 pitches, having allowed three earned runs in four innings.

Scherzer’s start on July 25 went smoothly, as he tossed six innings of one-run ball against the White Sox, but that followed an abbreviated two-inning outing in which he allowed four runs on July 20 against the Orioles.

Immediately after the St. Louis start, Scherzer said he did not expect to need a stint on the injured list, but the Rangers announced he would go on the IL just before Friday’s series opener against the Red Sox at Globe Life Field.

Pennington, 26, was acquired by Texas in exchange for Michael Lorenzen before the Trade Deadline. Pennington has made only one MLB appearance, retiring two batters for the Royals on July 5 this season. Pennington was 6-3 with two saves and a 2.20 ERA for Triple-A Omaha and Triple-A Round Rock this season. He threw 61 1/3 innings over 38 games, including two starts.

In his four years in the Minor Leagues, Pennington is 22-11 with 10 saves and a 3.52 ERA (95 ER/243 IP).