Per the team's official report, Tarasenko (injured hand) and O'Reilly (broken foot) will miss significant time. Tarasenko won't be re-evaluated for another 4 weeks, while O'Reilly is not expected to be re-evaluated for another 6 weeks.
With the NHL's trade deadline a little over 8 weeks away, that's not going to give the Blues a heck of a lot of time to work with to get them both healthy and into some games before March 3rd. Since both players are bona-fide stars in this league, I can't imagine that missing a month or two in the middle of the regular season will impact their overall performance too much down the stretch, but you never know what injury concerns can do to impact a player's trade value - especially as a deadline rental for the playoffs, where guys often play through injury. If they're already banged up coming in, that could take a few suitors off the table and lessen the bidding war, therefore reducing the cost of acquisition.