Backes was scratched for the second consecutive game due to the injury. He has been out since he took a hit to the head from Blackhawks defenseman Brent Seabrook in the third period of Game 2. Seabrook was suspended three games for the hit.
Backes had 27 goals, 30 assists, a plus-14 rating and an average ice time of 19:33 in 74 regular-season games. He had one assist in the first two games of the series.
The Blues lead the series 2-1.