Blues general manager Doug Armstrong made it official Saturday that Steen was placed on injured reserve retroactive to Dec. 21 with a concussion, the second of his career with the Blues.
Steen, who leads the Blues with 24 goals and 38 points, missed the game against the Calgary Flames on Monday and is out indefinitely.
"I say indefinitely because it could be day-to-day or week-to-week with a concussion," Armstrong said during the Blues' morning skate Saturday prior to facing the Chicago Blackhawks. "There's no set timetable.
"He's not going to be going on this road trip coming up [Sunday against the Dallas Stars and Tuesday against the Minnesota Wild], but he is exercising and we hope he has a speedy recovery. (With) concussions, I don't feel comfortable putting a timetable on (them)."
The Blues believe the root of Steen's concussion symptoms began with an early-game hit against the Ottawa Senators on Dec. 16 in which the Senators' Zack Smith was given a penalty for an illegal check to the head. The League did not review the play for any possible disciplinary action. Steen took another high hit from Douglas Murray after the whistle during the second period against the Montreal Canadiens on Dec. 19 and a final collision came two nights later against the Edmonton Oilers and former teammate David Perron.
Steen left that game after the second period and did not return. He returned to St. Louis and did not accompany the team to Calgary, then flew to Ann Arbor, Mich., where he visited a concussion specialist Thursday who determined the symptoms.
"What we wanted to do before we made any announcements, we wanted to give [Steen] some time to rest and then he went and saw a specialist that deals in these injuries on the 26th of December, came back with the report and the report is he is suffering from concussion-like symptoms or a concussion," Armstrong said. "We're going to want to take our time. We're hoping that it's not a long-term thing."
Steen previously missed 39 games during the 2011-12 season with a concussion.
"I think we'd be just as careful if it was his first one," Armstrong said. "Anytime you deal with a head injury, you want to be cautious and that's what we want to do with Alexander. We're hoping that it's not serious and he can get back here in the near future. Dealing with Alexander before and dealing with David Perron's (concussion in 2010-11), sometimes the initial response is they don't come back as quickly. … We don't want to overstate or understate the situation."
Steen signed a three-year extension prior to the game against the Canadiens and has been an integral part to the Blues' success in recent seasons that have vaulted them at or near the top of the NHL standings.
"He's such a big part of our team," said forward Jaden Schwartz, who has taken Steen's spot on the top line with David Backes and T.J. Oshie. "He plays in all situations, he's a big leader, so it's definitely tough. You want to make sure when he comes back, he's ready to go. We've got to have guys step up here and play more minutes than probably what they're used to and provide the work ethic that he has. We're going to have to have guys step up, but we're definitely going to miss him."
The Blues (24-7-5) will get forward Vladimir Sobotka (upper-body injury) back in the lineup Saturday night when they host the division rival Blackhawks (27-7-6). Sobotka has missed the past five games after being injured Dec. 14 against the Columbus Blue Jackets.
"I think getting Sobotka in helps out a lot," Blues coach Ken Hitchcock said. "That really eliminates probably the stress on other people that would have been there from an ice-time standpoint. You just adjust on the fly and go from there.
"We played without Backes, we played without Steen, we've played without Sobotka, without Schwartz … you just move ahead. It's a day-to-day injury (for Steen) so it's not like a three-month injury where you're sitting there, or like a two-month injury like it was with [defenseman Jordan] Leopold or [forward Ryan] Reaves. We'll just move ahead and see how we manage things."
The Blackhawks did not skate Saturday after beating the Colorado Avalanche 7-2 at home Friday night.
Here are the projected lineups for the Blackhawks and Blues: