NFLPA executive director DeMaurice Smith said Tuesday, "We are sufficiently concerned about what we've heard to begin an investigation."
Freeman said in a statement released by his agent late Monday that he voluntarily entered the substance-abuse program and submitted to random testing more than a year ago after he mistakenly took Ridalin instead of Adderall to treat Attention Deficit Hyperactivity Disorder.
The Bucs benched the fifth-year pro last week after Freeman played poorly in the team's first three games, all losses. The quarterback believes someone within the Bucs organization leaked information about him being in the league's drug testing program.
Smith was in town Tuesday to meet with Tampa Bay players as part of a league-wide tour to discuss player benefits and other union matters.
Meanwhile, coach Greg Schiano said he was "absolutely not" the source for an ESPN report that Freeman is in Stage I of the substance-abuse program.
Freeman's statement, released through agent Erik Burkhardt, said he has passed all 46 drug tests he's taken for the league since entering the program.
The quarterback said he is "prescribed and permitted to take medication" to treat ADHD for the entirety of his NFL career and that he has never tested positive for "any illegal drugs or related substances."
While Schiano said league rules don't permit him to comment on the situation, he did respond to questions about whether he was involved in leaking any information.
"I really don't want to get into what Josh's thoughts are about how things got out. I know what I've done, and I'm 100 percent comfortable with my behavior," the coach said.
"Certainly that's not what you want to happen at all. But, alluding to the accusations, I don't appreciate that either when someone's accused of something that's not true," Schiano said. "At the end of the day, it's not a good thing, and we need to just keep moving forward beyond it."
Smith did not discuss specifics, either.
"I will say this, we have a collective bargaining agreement that mandates and protects confidentiality and privacy." Smith said. "If we believe that any member of the team management or anyone from the league has deliberately taken steps to thwart that privacy and breach that confidentiality, this union will take every step and file every grievance and pursue any law to rectify that.
"I appreciate the manner in which Josh has handled this personally because he's a good young man. But this issue is a bigger issue about what's right with respect to the relationship between player and management," Smith added. "And when those issues come to bear, this is a union that will stand up for its players."
Freeman said random testing has been conducted at the Buccaneers training facility "because I spend all of my time there and I have nothing whatsoever to hide or be embarrassed about."