Sources familiar with the case tell us ... Frank's lawyers are researching and preparing legal docs so they can go to court and ask for a mandatory injunction, forcing the MLB and Selig to reinstate McCourt as the man in charge of the day-to-day operation of the team.
Our sources say the essence of McCourt's case -- that Selig was allegedly "arbitrary and capricious" in yanking McCourt's authority.
We're told one of the arguments -- that Selig and the MLB have tolerated bigger problems with other teams, including the New York Mets, which is reportedly $450 million in debt.
Our sources say legal action "is not imminent" -- meaning it won't be filed today. But sources say it could still be filed as early as Friday.