St. Louis, 38, will be going to his second Olympics, having played for Canada in 2006. It'll be his first international tournament since the 2009 IIHF World Championship, when he had 15 points in nine games.
"They called me yesterday," St. Louis said. "Great opportunity for me. I think if you asked Stamkos who he wants to replace him it would be me. So I'll do the best I can."
"It's great for Marty," Stamkos said. "If there is anyone in our locker room that deserves it from day one it is Marty."
The Lightning announced Wednesday that Stamkos has not been medically cleared to resume playing so he had to withdraw from the Olympics. Stamkos has been out of the lineup since Nov. 11, when he broke his right tibia in a game against the Boston Bruins at TD Garden.
"I don't see this as Marty replacing me. I see this as Marty getting the opportunity he deserves," Stamkos said.
St. Louis understood once the announcement about Stamkos was made that he would be a top candidate to replace him, so the news did not come as a surprise.
"Everybody that knows me knows I'm a pretty emotional guy. There has been a lot of that the last month. I heard from [Stamkos] that he wasn't going so I prepared myself," St. Louis said.
St. Louis is tied for 16th in the NHL in points this season with 54. He was considered to be of the top snubs when the Canadian roster was originally announced Jan. 7 and his name was not on the list. It was the second time in four years Hockey Canada general manager Steve Yzerman left him off the roster. Unlike the first time, Yzerman now serves as his GM in Tampa Bay as well.
Getting the call is yet another honor for St. Louis, who has gone on to an outstanding NHL career after going undrafted.
"I'm just glad to get the opportunity," St. Louis said. "I feel like my whole career has been a back-door entrance. This is no different. It's a big opportunity and I'm glad to be there."
He admitted there was disappointment over initially being left off the team.
"Of course I was bitter, but it wasn't motivation. I've been motivated for 10 years," St. Louis said. "If you are not motivated you are not considered for these things.
"As bitter as you can feel at times, you have to put that aside. You only get so many kicks at the can, so you take them."
One of those congratulating St. Louis on Thursday was Vancouver Canucks coach John Tortorella, who won a Stanley Cup with St. Louis in Tampa Bay. TSN's Amanda Stein reported he sent his former player a text with well wishes.
"They won't get him off the ice out there," Tortorella said. "He's that good. I couldn't be happier for him."
NHL.com correspondent Lonnie Herman contributed to this report.