O'Reilly, a center, has a broken foot and will be reevaluated in six weeks. The Blues captain played through the injury sustained after blocking a shot by Calen Addison during a 5-2 loss to the Minnesota Wild on Dec. 31. He was minus-2 in 19:20 of ice time, and has 16 points (10 goals, six assists) in 37 games this season.
Tarasenko, also a forward, will be reevaluated in four weeks for a hand injury sustained during the first period against the Wild after blocking a shot by Minnesota defenseman Matt Dumba. He had returned after missing one game with an illness, and has 29 points (10 goals, 19 assists) in 34 games.
"In years past, we've fought through injuries and found a way through it, and it's got to be the same story," Blues forward Robert Thomas said. "We've got a big challenge ahead of us and a lot of people are going to have to step up and I'm excited for the challenge for a lot of guys and we just got to fight through it."
General manager Doug Armstrong said he knew right away that Tarasenko was injured. He found out about O'Reilly on Sunday.
"I got the information that he came into the rink yesterday and wasn't feeling great," Armstrong said. "We sent him down to get some pictures taken and it came back a little bit more serious than we were hoping it was going to be.
"I think it's a great opportunity for a lot of different players. 've been able to touch base with some of our veteran guys via text that the reality is that some of the players aren't playing to the standards that they set for themselves the first 30-plus games. The challenge for them is to get their game back on par and lead us and then we have opportunity for younger players."
The Blues (17-17-3) begin a four-game road trip at the Toronto Maple Leafs on Tuesday (7 p.m. ET; SNO, BSMW, ESPN+, SN NOW).
"That's not the news you want coming into a road trip, your captain and your goal scorer going down, but at the same time, you don't dwell on who's out," forward Brayden Schenn said. "You have to focus on who's in and this is an opportunity for everyone to step up, eat more minutes, play better and find ways to win hockey games.
"We feel like we're coming along and if we get through this, and obviously it's a long stretch, but there's a chance for us to build something here, clean up defensively, we're going to have to know we're going to have to win games defensively and rely on special teams to win hockey games. No better opportunity than to start with a tough team in Toronto."
Forward Jake Neighbours was recalled from Springfield of the American Hockey League.
NHL.com independent correspondent Lou Korac contributed to this report
