As usual, the opening of free agency only means the Devils lose players.

The exodus from Lou Lamoriello’s happy family began yesterday, as early as it could, when late-season rental defenseman Sean O’Donnell signed what is believed to be a three-year, $7.5-million deal with the Bruins.

The Devils appear about to lose 43-goal scorer Alexander Mogilny, with the bidding believed to be among Detroit, Dallas, Toronto and the Rangers, while they might also lose defenseman Ken Sutton and center Bob Corkum to unrestricted free agency.

O’Donnell was acquired from the expansion Wild on March 4 for defense prospect Willie Mitchell. He played with and for Larry Robinson in Los Angeles, then played under Jacques Lemaire in Minnesota, then under Robinson with New Jersey. Now he will play for ex-Devils coach Robbie Ftorek in Boston.

O’Donnell, who made $1.1 million last season, was an immediate regular on the Devil defense until replaced by Sutton for Games 5 and 6 of the Stanley Cup Finals.