Dunn was hit in the face by a shot from San Jose defenseman Brenden Dillon in the first period of Game 3 on Wednesday. He left the game and did not return; the Blues are listing him as day-to-day.
"He's home and he's just recovering," Blues coach Craig Berube said after the morning skate.
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The Blues trail the best-of-7 series 2-1.
Gunnarsson, 32, comes back into the lineup after sitting out the first three games of the series. He also did not play the first four games of the first round against the Winnipeg Jets.
"I thought it was pretty quick [getting up to speed], for sitting out the first four," Gunnarsson said about his return to the lineup on April 18. "I didn't feel like I missed a step in that one, so hopefully it's the same thing tonight."
He has played nine playoff games this season without a point. In 52 Stanley Cup Playoff games he has three points, all assists.
"I'm really excited. It's going to be a lot of fun," Gunnarsson said. "Just been sitting on the side waiting for this so it's going to be great tonight."