The trade came hours after the Mets learned that Sanchez, a 26-year-old right-hander, had separated his right shoulder Sunday evening in Miami, when the taxi cab he was riding in was involved in a traffic accident.
Mets GM Omar Minaya said that Sanchez tried to guard himself in the collision with his right hand, which played a part in the injury.
"The bullpen was something I felt we were meaning to address, no matter what," Minaya said. "We were going to look for another piece of the 'pen. I just wish we would have had that piece with Duaner Sanchez there."
Sanchez returned to New York on Monday, where he has been hospitalized and will undergo further testing on his shoulder. If surgery is deemed necessary, Sanchez would miss the remainder of the season.
Hernandez, 41, could help fill the late-inning relief void left by Sanchez's absence. The right-hander pitched brilliantly for parts of last season with the Mets before leaving for Pittsburgh via free agency.
In 46 appearances for the Pirates this season, Hernandez was 0-3 with a 2.93 ERA and two saves in five opportunities. In 43.0 innings, Hernandez had allowed 46 hits and 24 runs (14 earned), walking 24 and striking out 33.
"Bert Hernandez is having a good year," Minaya said. "He's done it and he knows New York. We're fortunate to have him, given the circumstances."
Perez, a 24-year-old left-hander, was the Pirates' Opening Day starter in April, but had suffered through a horrid season that saw him demoted to Triple-A Indianapolis.
In 15 major league starts this year, Perez was 2-10 with a 6.63 ERA, allowing 64 runs (56 earned) and 88 hits in 76.0 innings.
"I can't tell you based on his recent performance that Oliver Perez is a guy we're going to count on," Minaya said. "But there's a lot of baseball left, and this kid could turn it around."
The Mets' Opening Day right fielder, Nady batted .264 with 14 home runs and 40 RBI, with his numbers somewhat impacted by injuries – he was sidelined in late May with an emergency appendectomy, and had also been fighting wrist issues in recent weeks.
With Nady's trade to Pittsburgh, outfielder Lastings Milledge is expected to re-join the Mets for the stretch drive, combining with Endy Chavez to help fill the Mets' right field needs.
"I think we're going to run more and be able cover more range in the outfield, which will make our pitching better," Minaya said.
Milledge, 21, had been involved in trade speculation regarding a number of pitchers leading up to Monday's trade deadline.
In a 24-game stint with New York earlier this season, Milledge batted .233 with three home runs and 12 RBI.
A .277 hitter in 76 games at Triple-A Norfolk, Milledge was ejected in the first inning Sunday after arguing balls and strikes with home plate umpire Mike Estabrook during a game against the Richmond Braves.
"I do believe Lastings is going to flourish," Minaya said. "This kid likes the challenge. I have a feeling he will step up."