Embedded video_content_type: Sochi 2014 hockey preview
Stamkos has been practicing and hoped to play for the Lightning on Saturday against Detroit in their final game before the Olympic break. He had 14 goals and nine assists in 17 games this season.
"I heard from Stammer that he wasn't going to go, so I knew it was a possibility," St. Louis told reporters in Tampa on Thursday. "So I guess I was prepared for it."
St. Louis said he felt bad for Stamkos.
"I think we've got to understand how hard he's tried and worked to put himself in the position he's in and give himself a chance," St. Louis said. "Obviously he's disappointed and I'm disappointed for him. Stammer's a true professional and he's done everything he can this past month to get back to the Lightning first and hopefully to Team Canada."
Steve Yzerman, the Canadians' executive director and Tampa Bay's general manager, chose St. Louis over Philadelphia's Claude Giroux and Pittsburgh's James Neal.
St. Louis had 54 points in 56 games, entering Thursday night's game against Toronto. The 38-year-old forward led the league in points in the 2012-13 shortened season with 17 goals and 43 assists in 48 games.
"I don't see this as Marty replacing me, I see it as Marty deserving a spot on this team and going over and hopefully bringing back a gold medal," said Stamkos.
St. Louis has won two World Championship silver medals for Canada over his career. He played in the 2006 Turin Olympics, scoring two goals and an assist in Canada's disappointing seventh-place finish.