Stallworth was sentenced in a Miami court Tuesday to 30 days in jail, a controversial ruling that drew a great deal of criticism.
Commissioner Roger Goodell wrote a detailed letter to Stallworth explaining the stiff penalty levied by the league.
"The conduct reflected in your guilty plea resulted in the tragic loss of life and was inexcusable," Goodell wrote. "While the criminal justice system has determined the legal consequences of this incident, it is my responsibility as NFL commissioner to determine the appropriate league discipline for your actions, which have caused irreparable harm to the victim and his family, your club, your fellow players and the NFL."
The suspension is effective immediately, although Goodell still plans to schedule a meeting with Stallworth to determine the length of the suspension. But now it appears the team will have no choice but to cut ties with the seven-year veteran.
Stallworth's lawyer, David Cornwell, released a statement late Thursday expressing confidence that Goodell would take Stallworth's acceptance of responsibility, and his cooperation with authorities and the family, into consideration.
"Consistent with the wishes of Mr. Reyes' family not to relive a tragedy through the public scrutiny of criminal and civil trials, we look forward to addressing these matters privately with commissioner Goodell," Cornwell said. "When [Roger Goodell] has the opportunity to review the 'unique facts involved,' ... we fully anticipate that the factors that supported the 'just resolution' of the criminal matter will be equally persuasive with Commissioner Goodell."
Browns general manager George Kokinis said in a statement: "We support the action taken by the commissioner today and will continue to work closely with the league regarding this matter."
The NFL Players Association had no comment.
In a memo sent to all 32 NFL teams, obtained by ESPN senior NFL analyst Chris Mortensen from a team source, Goodell sought to reinforce the league's policy on "alcohol-related misconduct."
"DUI is a serious matter which poses great risks to both those who drive
under the influence, and innocent third parties. This truth was tragically underscored in
Mr. Stallworth's case," Goodell wrote in the memo.
"In the past few years, I have not hesitated to impose discipline, including
suspensions, on club and league employees who have violated the law relating to alcohol
use. Every club should advise its employees of their obligations and our commitment to
hold people accountable for alcohol-related violations of law."
After a night of drinking at a bar in Miami Beach's Fountainebleau hotel, police said Stallworth hit the 59-year-old Reyes, a Miami construction worker, who was rushing to catch a bus after finishing work at about 7:15 a.m. Stallworth told police he flashed his lights in an attempt to warn Reyes, who was not in a crosswalk.
Stallworth had a blood-alcohol level well above Florida's legal limit. He stopped after the crash and reported the accident. Police estimated Stallworth was driving about 50 mph in a 40 mph zone.
Miami-Dade State Attorney Katherine Fernandez Rundle cited Stallworth's cooperation and willingness to accept responsibility as factors in the plea deal. Rundle also said the Reyes family -- particularly the victim's 15-year-old daughter -- wanted the case resolved.
Stallworth also must undergo drug and alcohol testing. His driver's license was suspended for life and he must perform 1,000 hours of community service.
Stallworth told Miami-Dade Circuit Judge Dennis Murphy that he hopes to get involved in drunken driving education programs.
"I accept full responsibility for this horrible tragedy," Stallworth said. "I will bear this burden for the rest of my life."
Stallworth signed a seven-year, $35 million contract with the Browns before last season but was injured much of the year, finishing with 17 catches for 170 yards and a touchdown. A star at the University of Tennessee, Stallworth has also played in the NFL for New England, Philadelphia and New Orleans.
The night before the crash, Stallworth earned a $4.5 million roster bonus from the Browns, whose offseason moves since have indicated they were not counting on having him available.
They added free-agent wide receiver David Patten this spring and then drafted Ohio State's Brian Robiskie and Georgia's Mohamed Massaquoi. Patten is in his second stint with the club after stops with New Orleans, the New York Giants, New England and Washington.
The Browns also released receiver Joe Jurevicius three days before the accident.
Stallworth began serving the sentence Tuesday. He also was sentenced to two years of house arrest following his release from jail, and will be on probation for eight years. He had faced 15 years in prison.
But Goodell showed no mercy in the wording of his letter to Stallworth.
"There is no reasonable dispute that your continued eligibility for participation at this time would undermine the integrity of and public confidence in our league," he said.
And in his memo to the teams, Goodell made his message quite clear: "Let's make sure that the 2009 season does not bring more tragedy or embarrassment to ourselves and our employees."
James Walker covers the AFC North for ESPN.com. Information from The Associated Press was used in this report.