Phaneuf boarded Miller, who clearly had his back turned, during Toronto's loss to Boston on Sunday night at Air Canada Centre. No boarding penalty was called on the play.
Miller left the game, but Claude Julien told reporters that he wasn't worried about the defenceman's condition.
Phaneuf had a phone hearing with NHL vice-president of player safety Brendan Shanahan on Tuesday morning. That limited the possible suspension to five games or fewer.
The suspension is the Leafs' fifth this season. David Clarkson got an automatic 10 games for leaving the bench to join an altercation during the pre-season, Phil Kessel got three pre-season games for slashing John Scott during that brawl with the Sabres and Nazem Kadri got three games for elbowing Wild goaltender Niklas Backstrom in the head last month.