The righty was hit hard Sunday in a 14-2 loss to the Miami Marlins, his second straight bad outing, then admitted to reporters after the game that he was pitching hurt.
"It's not something that I had before," Halladay said Sunday. "It's something new this year. I felt good all spring. I had soreness in there and just wasn't able to get rid of it."
Halladay, who turns 36 this month, is 2-4 with an 8.65 ERA in seven starts this season.
Chicago White Sox pitcher Gavin Floyd will have season-ending Tommy John surgery on his right elbow, and will likely miss 14 to 19 months.
The right-hander, who was hurt April 27 outing against the Tampa Bay Rays, will have surgery on Tuesday, after receiving multiple opinions.
Floyd went 0-4 with a 5.18 ERA in five starts this season.
The Boston Red Sox placed closer Andrew Bailey on the disabled list due to right biceps inflammation. Bailey, who has five saves and a 1.46 ERA, received an MRI that revealed no structural damage. He last pitched April 28.
Left-hander Craig Breslow, who hasn't pitched yet season, was activated from the DL to take Bailey's spot on the roster.
Injured New York Yankees third baseman Alex Rodriguez returned to the field Monday for the first time since having hip surgery in January.
The 37-year-old ran, played catch and hit off a tee with other injured starters at the team's minor-league complex.
Rodriguez could return to the Yankees after the All-Star break. He had surgery Jan. 16 to repair a torn labrum and impingement in his left hip.
The Chicago Cubs purchased the contract of outfielder Ryan Sweeney and recalled pitcher Rafael Dolis from Triple-A Iowa, while sending outfielder Dave Sappelt to the minors and designated pitcher Kameron Loe for assignment.
Sweeney, 28, was batting .337 at Triple-A level. He signed with the Cubs on April 2 after being released by the Boston Red Sox at the end of spring training.
Atlanta Braves catcher Brian McCann was activated from the disabled list Monday, after missing the season's first five weeks following surgery on his right shoulder.
Infielder Tyler Pasternicky was optioned to the minors to make room on the roster.
The St. Louis Cardinals optioned infielder Jermaine Curtis to Triple-A Memphis to make room for Matt Adams' return.
The Cleveland Indians announced they placed right-handed pitcher Vinnie Pestano on the 15-day disabled list with right elbow tendinitis. They also recalled left-handed pitcher Nick Hagadone from Triple-A Columbus.
Former Major Leaguer Otis Nixon was arrested over the weekend after police found him in possession of crack cocaine, the Atlanta Journal Constitution reported.
Nixon, 54, had a crack pipe in his pocket and a crack rock in his vehicle when he was stopped early Saturday, according to a Cherokee County Sheriff's Office reported obtained by The Atlanta Journal-Constitution.
Chicago White Sox pitcher Jake Peavy was scratched from start on Monday because of back spasms, but is scheduled to start on Wednesday against the New York Mets.
Peavy is 3-1 with a 3.38 ERA.
Left-handed pitcher Francisco Liriano is expected to make his Pittsburgh Pirates debut on Saturday, the the Pittsburgh Post Gazette reported. He missed the first six weeks of the season with a broken eight arm.