Tatis will undergo further testing on Tuesday at New York's Hospital for Special Surgery.
"He's had some problems with it since even Spring Training," manager Jerry Manuel said. "That has a lot to do with the fact that he wasn't able to get the bat in a position of strength with that shoulder. I guess the doctor looked at it today and decided that's the best thing for him."
Tatis, 35, was batting .185 almost exclusively as a pinch-hitter. He had recorded only two hits since May 23, one of them a bloop double.
Valdes, 32, posted a 5.10 ERA in 30 innings earlier this season -- somewhat of a disappointment after he fashioned a 1.86 ERA in April.
The Mets, primarily, are interested in his fresh left arm. After using five relievers in Sunday's 9-5 victory over the Nationals, two on Saturday and four on Friday, the team desperately needed a pitcher capable of throwing multiple innings. Valdes, who pitched at least three innings in four of his 20 appearances with the Mets, fits that bill.
"We needed someone that could give us some length," Manuel said. "He's a guy that's been here and has done a decent job."
The Mets made one other move to shore up their bullpen, altering their rotation to bump Hisanori Takahashi from his final start of the first half. In place of Takahashi, Mike Pelfrey will start on regular rest on Saturday. Johan Santana will start Sunday.
Takahashi, who threw 86 pitches in Sunday's game, will be available out of the bullpen starting Wednesday.
-- Anthony DiComo