Hernandez, who went 7-8 with a 5.47 ERA for New York, will be in the rotation and serve as a mentor for Washington's young starting pitchers.
In fact, he will start against the Cubs on Wednesday night, while right-hander J.D. Martin and left-hander John Lannan will be pushed back a day and pitch on Thursday and Friday, respectively.
"We are running out of innings with some of our young guys, and I refuse to push them past their limits," general manager Mike Rizzo said. "Livan was available, and I talked about him with our player personnel people. He is a good guy in the clubhouse. He has some friends here on the ballclub. He is nothing but a workhorse and a reliable arm to get some innings and get us through the rest of the season."
This will be Hernandez's second stint with the Nationals/Expos. He played for the organization from 2003 to the middle of the 2006 season, when he was traded to the D-Backs for right-hander Garrett Mock and left-hander Matt Chico.
"His locker is going to be next to mine," Mock said. "I think Livan is an unbelievable pitcher. He has [a World Series] MVP Award. He seems to perform. It's going to be a pleasure to watch the guy up close, watch him work and attack hitters. I'm going to pick his brain a little bit. It's exciting."
Hernandez's best season with the organization was in 2003, when he was 15-10 with a 3.20 ERA for Montreal. He also led the National League in innings pitched from 2003-05.
Hernandez, 34, has played 14 years in the big leagues, going 154-147 with a 4.43 ERA for the Marlins, Giants, Expos, Nationals, D-Backs and Mets.
To get Hernandez on the 40-man and 25-man rosters, the Nationals optioned Collin Balester to Triple-A Syracuse and transferred first baseman Dmitri Young from the 15-day disabled list to the 60-day DL.
Balester went 1-4 with a 6.82 ERA in seven starts, last pitching on Monday against the Brewers. He cruised through the first five innings, but allowed four runs in the next inning.
"We can't forget that Collin is 23 years old and he has a bright future ahead of him," Rizzo said. "His stuff is second to none. He just needs to put it all together and be more consistent."
Young has yet to play a Major League game in 2009 because he is out for the season due to a torn left quad.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.