The Falcons will receive no return on the guaranteed $2.5 million salary in 2008 for the 36-year-old Horn, who said he wished he could have produced better numbers for team owner Arthur Blank.
"It's a sense of sadness because after every game last year and after the first two preseason games I had to look at Mr. Blank and know that he deserves a winning team and he deserves a championship," Horn told The Associated Press. "I was here and couldn't make it happen. It's kind of sad for me leaving."
Horn was a disappointment in 2007, when he caught only 27 passes for 243 yards and one touchdown after signing a four-year, $14.5 million deal.
Horn sat out voluntary workouts, did not play in the team's first two preseason games because of a hamstring injury and asked to be traded.
"I'm 100 percent ready to run," Horn said Tuesday when asked about the injury.
Horn's agent, Ralph Vitolo, said Dallas, Jacksonville, Seattle and Tennessee could have a need for a veteran receiver.
"Now we're calling these teams and kind of narrowing the scope," Vitolo told AP. "Joe has been so productive. Somebody like Joe and a team with a need should be a perfect fit."
Vitolo called Horn's hamstring injury "just a nagging thing."
"It was just mentally wearing him down more than physically," he said.
Horn, drafted by Kansas City in 1996, spent seven years with the New Orleans Saints. His production declined his last two years with the Saints, but still the Falcons offered $7.5 million in guaranteed salary and bonus when looking for help at wide receiver in Bobby Petrino's pass-oriented offense last season.
"That's what locked him in with Atlanta," Vitolo said of the guaranteed money.
Petrino lasted only 13 games as coach before quitting with three games left in the 2007 season. The Falcons now are rebuilding with coach Mike Smith and a run-oriented attack.
"I'm disappointed that Joe didn't get an opportunity to compete because of his injury in training camp and his choice not to come to OTAs," Smith said after Tuesday's practice. "We really didn't get an opportunity for him to compete. We're going to move forward. We feel very comfortable about the receiving corps that we have. I wish Joe the best of luck as he moves on."
Horn had only five catches for 40 yards in the final three games last season.
"Everything imploded," Vitolo said of the 2007 season. "We couldn't change something we couldn't control. The Petrino thing imploded on us. It wasn't the Falcons' fault and it wasn't Joe's fault. It just all fell apart. He planned on retiring with Atlanta."
"That's what happens in this league," Horn said. "You play awhile and youth comes along. It's their time. You don't rebuild a team with veteran players. I do think you can rebuild with a veteran player who can still play. I know I can still play."
Horn has 603 career receptions for 8,744 yards and 58 touchdowns. His last of four 1,000-yard seasons was 2004, when he had 94 catches and set career highs with 1,399 yards receiving and 11 touchdowns with the Saints.
The Falcons also placed linebacker Isaac Brown, an undrafted rookie free agent from Central Michigan, on the waived/injured list.