KANSAS CITY -- Tyler O'Neill is headed back to the injured list.

Boston’s slugger, who had missed the past four games due to an undisclosed general illness, was placed on the 10-day IL (retroactive to Aug. 4) pregame Wednesday with a left leg infection. Manager Alex Cora said the infection popped up during the club’s final game in Texas on Sunday.

“[O’Neill’s] been sick, and actually toward the end of the trip in Texas, he had this infection in his left leg. It’s gotten worse and he’s getting treatment,” Cora said. “He won’t be able to play in the next, probably three or five days. We’ll see how it goes.

“So we decided to put him on the IL. We got [Enmanuel Valdez] here, a left-handed bat, we can mix and match later in the game. He can play second, he played left, so I like where we are at roster wise.”

The club is unsure if the illness that had kept O’Neill out for the past four games was related to his leg infection, but the Red Sox expect him to return after the 10-day minimum stint. The club recalled Valdez, who is batting .226 with six homers in 56 games this season, as the corresponding move.

“I talked to [O’Neill] and he said, ‘Alex, I have no idea what happened,’” Cora said. “When J.J. [athletic trainer Jon Jochim] saw him in Texas, he was in bad shape. Then I think it was the last day that his leg was huge. He’s been getting treatment, doing everything possible with the medical staff to take care of this. … It just happened. He doesn’t know. He woke up in the morning and [his leg] was big.”

O’Neill’s third stint on the IL this season is a big blow to a Boston offense that has taken off since the All-Star break. The right-handed slugger has crushed 22 homers and driven in 45 runs while compiling a .901 OPS in 80 games this season. Since Boston traded for O’Neill in December -- sending RHPs Nick Robertson and Victor Santos to St. Louis -- the 29-year-old has landed on the IL with right knee inflammation and a concussion.

The Red Sox also received tough news Wednesday with starter Nick Pivetta. Cora said the club will skip the right-hander's next start because of low metrics and how he was responding to his last start. Pivetta, who has thrown at least 142 innings in each of the past three seasons, has a 4.44 ERA over 95 1/3 frames this year.

Pivetta missed parts of April and May when he was placed on the IL with a right elbow flexor strain. In his last start Sunday against Texas, the 31-year-old averaged 91.9 mph with his fastball, which was down from his season average of 94.1 mph.

“He’s not responding well out of this [start]. We do believe that skipping one, using the off-day, we’re going to reset him and he should be fine,” Cora said. “We don’t like the velocity of the fastball. The other pitches were OK, but we have to take care of him.”

The Red Sox did receive some positive news Wednesday, though, with veteran reliever Chris Martin feeling well enough to skip a rehab assignment and be activated from the 15-day IL for the series finale against Kansas City. The 38-year-old, who had a 1.05 ERA over 55 games with the Red Sox last season, had a 3.42 ERA in 26 games before hitting the IL with right elbow inflammation. To make room for Martin, the Red Sox optioned lefty Cam Booser, who had a 2.77 ERA in 39 innings this season, to Triple-A Worcester.