Rossum was surprised by the move when summoned to a meeting with coach Mike Singletary. Rossum also spoke with general manager Scot McCloughan.
Rossum, 33, ranks second in NFL history with 14,987 return yards. He hadn't been active the past two games after playing in the first three to start his second season with the 49ers.
"Unfortunately, as this business goes, sometimes decisions are made that you don't understand, but you have to live with it and they have to live with it as well," Rossum said in the parking lot before leaving the team's facility. "I don't think I've disappointed anybody when I've stepped on the field."
Rossum had packed a few belongings and left team headquarters before the 49ers got to work during their bye week. He plans to return to his family in Atlanta, where he will decide whether to try to join another team or retire after parts of 12 NFL seasons.
"If my time has come, my time has come," he said. "My utmost passion is my family."
Rossum will be replaced by Crabtree, the 49ers' top draft pick in April and who finally signed a six-year deal last week. Crabtree will spend the bye week at team facilities while others get a break.
"He already had his bye week, a long bye week," Singletary said.
Singletary called Rossum "a joy to be around, an asset to the team."
"Sometimes the numbers don't quite work," the coach added. "I'm really thankful we had him and I'm thankful I got to know him. He'll do well."
Rossum was coming off one of his best seasons in 2008, having set career highs of 26.8 yards per kickoff return and 14.9 per punt return. The kickoff average ranked third in the NFC, and the punt mark was second best in the NFL.
"I think teams have to approach our return game differently when I'm on the field," Rossum said. "These last two weeks before every game, they've been shocked that I'm not up. I know I'm doing something good when those teams approach me and say, 'Hey, man, we've been preparing for you all week.' I'm not worried about it."
Rossum said he believes the 49ers (3-2) are on the upswing following six consecutive losing seasons. And despite his ouster, he respects the organization.
"As an NFL player you have to feel you're best whether you play or not," he said. "So I said, 'I know it's a tough decision for you because there's no way you can sit up there and say I shouldn't be on the field.' I respect them for releasing me now so I can get in somewhere else or I can just go home and spend time with my kids."
Copyright 2009 by The Associated Press