"It was news to us at the time," Cardinals manager Mike Matheny said. "We let him get it looked at, and he's going to continue to get some more tests [done]."
Rosenthal posted 45 saves in 2014 and set the franchise record for saves last season by closing out 48 games, but he was removed from the ninth-inning role this season after struggling to consistently record outs. In 40 appearances this season, he has a 5.13 ERA and a 2.04 WHIP.
His BABIP of .447 this season is the highest in the Major Leagues (minimum 30 innings pitched), and he's more than 50 points higher than the Angels' Jose Alvarez, who owns the second-worst mark in the league.
"I think this is similar to when Seth Maness [was placed on the DL with right elbow inflammation] and things just weren't lining up," Matheny said. "[Maness] wasn't necessarily feeling anything that was alarming, but he wanted to get it looked at and figure things out. I think [Rosenthal's situation] is along those same lines, and hopefully, this will be a step in the right direction."
In a corresponding move, the Cardinals recalled left-hander Dean Kiekhefer from Triple-A Memphis, and St. Louis will also recall right-hander Sam Tuivailala to serve as the club's 26th man for the second game of Tuesday's doubleheader against the Mets.
Kiekhefer has allowed runs in three of his 11 appearances with St. Louis this season after making his Major League debut on May 14. He is 4-1 with a 2.00 ERA in 23 Triple-A relief outings.
Tuivailala has pitched two innings in three relief appearances with the Cardinals this month, and he pitched 15 2/3 innings with St. Louis in 2014 and '15.
Troy Provost-Heron is a reporter for MLB.com and covered the Cardinals on Tuesday.