The Norris Trophy nominee was hit by Chicago's Troy Brouwer with about 8 minutes left in Saturday's Game 5, which the Blackhawks won 5-1 to take a 3-2 series lead. Phaneuf skated off the ice after the hit and did not return.
"Our heads did collide and I think him not ready for it, he's not able to brace himself and that's why it stunned him I guess," Brouwer said Monday.
Flames coach Mike Keenan said Phaneuf "is not available" for Game 6 and did not participate in the team's Monday skate. Anders Erikkson, who played in the first game of the series, was expected to replace him in the lineup and defenseman Matt Pelech was also expected to dress.
Phaneuf led the Flames in hits during the regular season and his powerful slapshot from the point is an asset on Calgary's power play. The 6-foot-3, 213-pound defenseman had three assists in the first five games of the series.
Phaneuf was responsible for the slapshot in the first period of Game 5 that disfigured the finger of Chicago defenseman Matt Walker.
Walker eventually returned to the game and, although he skipped Monday's skate, Blackhawks coach Joel Quenneville said Walker will play in Game 6.