Letang, 29, will miss the remainder of the regular season and the playoffs, with an expected recovery time of four to six months.
"Kris underwent an MRI in February that revealed a herniated disc in his neck," Penguins general manager Jim Rutherford said in a statement.
"Our neurosurgeons examined him and recommended conservative treatment in an attempt to correct the condition without surgery. Kris had been making progress and was hopeful of returning for the playoffs until about a week ago, when the progress plateaued. He underwent another MRI, and, at that point it was determined that surgery was needed."
Letang has not played since Feb. 21, missing the Penguins' last 20 games. The native of Montreal contributed 34 points (five goals, 29 assists) in 41 games this season.
In 11 NHL seasons, all with the Penguins, Letang owns 386 points (87 goals, 299 assists) in 603 games.
Letang is a two-time Stanley Cup champion. He played in the NHL All-Star Game in 2011, 2012 and 2016.
- - -
The Sabres also recalled Linus Ullmark from Rochester of the American Hockey League, one day after fellow goaltender Anders Nilsson was injured in practice.
Okposo collected 19 goals and 26 assists in 65 games after signing a seven-year, $42 million deal in the offseason. The 28-year-old power forward had recorded three straight seasons of at least 50 points with the New York Islanders.
- - -
The Columbus Blue Jackets signed defenseman Ryan Collins to a three-year, entry-level contract.
The team announced Collins will join the American Hockey League's Cleveland Monsters prior to Friday's game at Milwaukee.
(Editing by Andrew Both)