The suspension followed an investigation into the July 14 transaction in which pitcher Drew Pomeranz was sent to the American League team, a statement from the commissioner's office said.
According to previous reports, Red Sox officials discovered after the trade that Pomeranz had been receiving medical treatment, and that the Padres had not been fully entering player health data in a central database, which would give them an advantage in trade talks.
Pomeranz was an All-Star for his strong first half of the season in San Diego, but has compiled a won-loss record of 2-5 in 11 starts in Boston, with a relatively high earned-run average of 4.60.
“I accept full responsibility for issues related to the oversight of our medical administration and record keeping,'' Preller said in a statement released by the Padres.
“I want to emphasize that there was no malicious intent on the part of me, or anyone on my staff, to conceal information or disregard MLB's recommended guidelines,'' Preller said. “This has been a learning process for me. I will serve my punishment and look forward to being back on the job in 30 days.''
The Pomeranz trade was one in a flurry before baseball's non-waiver trading deadline in which Preller tried to remake a faltering organization and inject talented youth into the minor league system. The Padres also made a splash with several signings of international players.
Pitcher Colin Rea, who was sent to Miami in July in a multi-player deal, was returned to the Padres after he suffered an elbow injury in his first start with the Marlins. The Padres had to give up Luis Castillo, a prized minor league pitcher considered to be a top acquisition in the trade.
“We accept the discipline handed down from Major League Baseball earlier today and will fully comply with Commissioner (Rob) Manfred's recommendations pertaining to changes with our medical administration and record keeping,'' Padres Executive Chairman Ron Fowler said in the team's statement.
“Rest assured, we will leave no stone unturned in developing comprehensive processes to remediate this unintentional, but inexcusable, occurrence,'' Fowler said. “To be clear, we believe that there was no intent on the part of A.J. Preller or other members of our baseball operations staff to mislead other clubs.''
He said he was disappointed to lose Preller's services for a month, but will work closely with him upon his reinstatement “to ensure that this unfortunate set of circumstances does not happen again.''
Preller previously was suspended while working as the head of international scouting for the Texas Rangers, for talking to an agent about a player suspended for lying about his age and identity. He's completing his second season as the Padres' GM.