The team said Vokoun left the ice at practice at Consol Energy Center after noticing swelling in his thigh. He was taken to the emergency room, where doctors diagnosed the blood clot.
"The most important thing right now is Tomas' health and well-being," Penguins general manager Ray Shero said. "The clot was dissolved by the procedure, and the doctors tell us that he will remain in the hospital for several days. We will continue to monitor his progress after that."
GAA: 2.45 | SVP: 0.919
Vokoun could not finish the 2005-06 season with the Nashville Predators and was diagnosed with pelvic thrombophlebitis, a rare blood condition that created a multitude of blood clots, according to The Associated Press. Doctors at the Mayo Clinic cleared him prior to the 2006-07 season.
Vokoun played 20 games last season, going 13-4-0 with a 2.45 goals-against average. He was 6-5 with a 2.10 GAA in 11 Stanley Cup Playoff games.
The 37-year-old has played 15 NHL seasons with Montreal, Florida, Washington, the Predators and Penguins.
The Penguins lost to the Columbus Blue Jackets, 5-3, in a preseason game Saturday.
Shero said Vokoun is resting comfortably. The GM also said he expects Fleury to face more pressure with Vokoun out.
"Tomas was undergoing the procedure during the game," Shero said. "[Fleury's play] has been OK. It's been a work in progress. If Tomas is out for a little while, it's going to be a situation where Marc's going to have to step up."
NHL.com correspondent Wes Crosby contributed to this story.