Leopold will have surgery later this week after suffering a hand injury in a 3-2 victory against the Calgary Flames on Thursday.
Leopold has ligament damage in the knuckle on the first finger of his right hand and will be sidelined a minimum of two months. It is not known whether the injury was the result of a blocked shot or some sort of contact from a check in the defensive zone.
"He's going to be operated on this week, and we'll reevaluate him in two months and see how he goes," Blues coach Ken Hitchcock said after practice Monday. "In eight weeks we'll take a reevaluation of the hand and go from there.
"He has a ligament issue, need to get it solved right now so that he's back playing during the season. We'll get him looked at here and have that operation done as quickly as possible, and then he'll get back playing in hopefully two months."
Leopold has two assists in 13 games this season. An 11-year veteran, the Blues acquired him from the Buffalo Sabres at the NHL Trade Deadline last season for a pair of draft picks.
In the meantime, the Blues will at some point recall a defenseman from the Chicago Wolves of the American Hockey League and use Ian Cole, the 18th pick in the 2007 NHL Draft, as their sixth defenseman.
"He's a good player," Hitchcock said of Cole. "He's really helped himself here by his attitude and everything. He's ready to go. I don't see us missing a beat.
"We'll look at what we're going to do for the seventh [defenseman]. There's a couple guys in Chicago really starting to play well. We're going to look at that and go from there. This one will be up to [Blues general manager Doug Armstrong] and the scouting staff and stuff like that."
Cole, 24, skated with Roman Polak Saturday and played 13:26 in a 2-1 victory against the Penguins on Saturday. He's played in three games this season, with no points and is a plus-2. Cole has two goals and 11 points in 70 career games spanning four seasons.
"There has been, at times, such long periods between each game that I've played, that the mental side of it, as far as trying to keep up with the pace that it takes to play in the NHL is the hardest part," Cole said. "You’re still skating every day, you're still working out every day, you're in great shape … the legs feel fine. But as far as being able to make plays in situations that you don't get into in practice that are so foreign when you're not playing, that's kind of the hardest part. When you do play more, you get into those situations and you're much more able to adapt to those ever-changing situations."
Also for the Blues, veteran forward Brenden Morrow was activated from injured reserve after missing the past five games with an upper-body injury.
Morrow, 34, skated on the team's fourth line with Maxim Lapierre and Ryan Reaves, and Morrow got through Monday without any discomfort. He could be inserted back into the lineup against the Phoenix Coyotes on Tuesday or the Colorado Avalanche on Thursday.
"Yeah, back in the routine and I would expect to be back in the lineup tomorrow," Morrow said after practice Monday. "I think the last two or three days, just tried to push it a little bit more each day and see if there's any setbacks. Nothing I've done has bothered me. I guess you can never tell what's going to happen until you're in a game. You do your best to mimic it in drills and test it, but you're never going to be able to react until you're in a game situation. That's the next step."
Forward Chris Porter was assigned to the Chicago Wolves of the American Hockey League.