Zbynek Michalek is a defense who plays "shut down" minutes with a positive SAT (or corsi). Here is Arizona's player usage chart showing Michalek's on ice usage.
Michalek is second in corsi for percentage (51.8%) and one of three Coyote defensemen that are above 50%. He and the rest of the Coyote defense have sub 1000 PDO. He only generates .5 points per 60 minutes of 5v5 ice time and only 2 goals total so far this season. If the Blues were looking for a potential Shattenkirk replacement while he recuperates, I don't think this is it. But are the Blues missing a shut down defenseman right now? They sure are and Michalek could fit that need. The big asterisk to all these stats those is that these are pre-concussion which has sidelined him since February 14th. Armstrong is definitely taking a gamble considering he already had a defenseman out with an injury and now he picked up one that has a concussion. Not a big bold trade, but definitely risky. However, if Michalek returns to his pre-concussion level of play it could pay off for the Blues.