ARLINGTON, Texas -- Three-time Cy Young Award winner and active career strikeout leader Max Scherzer was placed on the 15-day injured list Friday because of right shoulder fatigue.

Scherzer (2-4) has made only eight starts since his season debut June 23 following offseason back surgery and then dealing with a nerve issue in his arm.

The Rangers announced the IL move before their homestand opener against Boston, after manager Bruce Bochy had spoken to reporters. The move was retroactive to Wednesday, a day after Scherzer didn't pitch past the fourth inning for the third time in a span of four starts.

Texas recalled left-handed reliever Walter Pennington from Triple-A Round Rock. He had been assigned to that minor league club after the Rangers acquired him Monday in a trade that sent right-hander Michael Lorenzen to Kansas City.

In his last home start July 25, two days before this 40th birthday, Scherzer allowed one run and struck out nine over six innings against the Chicago White Sox. He passed fellow three-time Cy Young winner Justin Verlander for 10th on the strikeout list -- and the most among active pitchers.

Scherzer, who was born in St. Louis and grew up in the area, allowed three runs in four innings while throwing 68 pitches in a loss against the Cardinals on Tuesday night. His five strikeouts pushed his career total to 3,405.

Bochy said after that game that Scherzer's arm "just fatigued there," and that he could feel it in his triceps.

Scherzer said that night that he was "trying to grind through these starts and ramp up." He added that he just needed an extra day and didn't need to go on the IL.

The Rangers had an off day Thursday, so had they stayed in order, his next turn would have been Monday. Now the earliest he would be available again is Aug. 15.

Losing Scherzer puts the Rangers rotation in more flux. Jon Gray is on the IL for the second time this season with a right groin strain, after getting hurt on his final warmup pitch before his scheduled start last Sunday, and the earliest he could return is Aug. 13. That came a day before they completed the deal to trade Lorenzen.

Tyler Mahle could make his Rangers debut Tuesday, nearly 15 months after Tommy John surgery. Texas signed the right-hander to a $22 million, two-year contract in free agency last winter with the anticipation that he could pitch this season.

Two-time Cy Young Award winner Jacob deGrom is scheduled to throw another bullpen session Saturday, and will progress to throwing to hitters if all goes well then. He is recovering from his own right elbow surgery, which came after he went 2-0 with a 2.67 ERA in his only six starts early last season after signing with Texas in free agency.