LeMahieu had been dealing with the injury for several weeks but managed to play through the discomfort until recently, missing the first three games of New York's series against Minnesota. Yankees manager Aaron Boone believes the issue really began to impact LeMahieu during a three-game set at Boston from Aug. 12-14.
One of New York's most consistent hitters, the two-time batting champion was 0-for-12 last weekend in Tampa Bay and has scuffled at the plate since that weekend at Fenway Park, batting just .143 (10-for-70) with zero extra-base hits over his past 18 games.
"It's just been sore and I think it's just prevented him from really getting off his swing," Boone said. "I talk about these guys, they're like race cars. A little something's off and it prevents you from being that race car."
The Yankees had hoped the reliable LeMahieu could avoid the IL. That outlook changed on Thursday, with Boone noting that treatment and playing through the pain simply hasn't worked well enough.
"There's pain things. We've talked about the orthotics he's got. We talked about the treatments that they do on it every day," Boone said. "Are there other injections that are safe and effective? These are all things that they're talking about."
The move is retroactive to Sept. 5, opening up the possibility that LeMahieu could be back next Friday at Milwaukee. While Boone remains cautiously optimistic, he did not set a timetable for LeMahieu's return.
"I'm concerned about it obviously because DJ's been dealing with this now for a few weeks where I think it's clearly compromised him," Boone said. "So yeah, there's that concern but there's also hope with that."
New York has been hampered by a rash of injuries to key players lately.
First baseman Anthony Rizzo landed on the IL Wednesday with headaches that followed an epidural injection to treat lower back pain.
Slugger Giancarlo Stanton is still dealing with the after effects of fouling a ball off his left foot in the series opener Monday. He was expected to participate in a full pregame routine Thursday and could be available over the weekend when the second-place Rays visit the Bronx.
Matt Carpenter was seen rolling through the clubhouse Thursday on a scooter, his broken left foot in a boot.
Luis Severino (strained right lat) allowed two runs on four hits over four innings Wednesday four Double-A Somerset. The right-hander might make one more rehab start but if not is a possibility to return for a two-game series at Boston Sept. 13-14, Boone said. A former 19-game winner, Severino last pitched in the majors on July 13.
Trade deadline acquisition Lou Trivino is dealing with back spasms, something the reliever noted usually takes two to three days to resolve. Right-hander Scott Effross (strained right shoulder) is expected to throw a bullpen session either Friday or Saturday and could be close to returning.
All-Star Nestor Cortes (strained left groin) was activated from the 15-day IL and started Thursday's game against the Twins. He threw 58 pitches in four innings, allowing two hits and two earned runs in the 4-3 loss to Minnesota.
The Associated Press contributed to this report.