It was another sudden blow in a tumultuous week for the 49ers, who saw defensive lineman Kentwan Balmer leave the team Monday. Balmer, the team's first-round draft pick in 2008, missed practice for the fifth straight day Friday. His absences have been unexcused since Wednesday.
"We just respond to this and move forward," coach Mike Singletary said. "We're dealing with this in stride and continuing to have our best practice. The thing that we're trying to do as the 49ers is that we're trying to find 53 men that love the game of football."
Coffee apparently isn't one of those. In a statement released by the team, the second-year veteran said, "I feel it is best for me that I move on from football."
Coffee, San Francisco's third-round draft pick in 2009, was the team's second-leading rusher as a rookie last season with 226 yards, but he averaged just 2.7 yards per carry and scored one touchdown.
Coffee was in competition this summer with rookie sixth-round draft pick Anthony Dixon to be the team's top backup to Pro Bowl halfback Frank Gore. The 49ers are looking to ease the workload this season on Gore, who is coming off his team-record fourth consecutive 1,000-yard rushing season.
Singletary, who said Coffee "was having a fine camp," said Coffee's abrupt decision caught the team by surprise.
Coffee didn't show up for Thursday's afternoon practice and then informed Singletary of his decision Friday morning.
"I appreciate his honesty and I appreciate him not coming out here and going through the motions," Singletary said. "He said his heart isn't in it. It's his decision. He is a man, and the last thing that I'm going to do is disrespect him and try to drag something out."
Coffee's departure leaves the 49ers with Dixon and fifth-year veteran Michael Robinson as the only halfbacks on the roster behind Gore. Singletary said the 49ers will immediately look to bring in another running back.
Robinson averaged 4.7 yards per carry as a backup to Gore in 2007 but has seldom been used as a running back the past two seasons. Robinson is San Francisco's special teams captain and a standout performer on those units.
"We had no indication this was coming," Robinson said. "Glen's a good friend and it's like I'm losing a brother. With Glen not being here, there is a void. But I've been here five years and I've seen backs come in and I've seen backs leave. We all have to elevate our games now."
The 49ers had been bringing along Dixon gradually, but his learning curve now will accelerate. Dixon will be Gore's top backup in Sunday's exhibition opener at Indianapolis.
Singletary said Dixon is "going to have to grow up real quick." Dixon's workload will increase and he will start seeing more snaps in practice.
Dixon has been eager to get his chance since he was drafted. The 233-pounder from Mississippi State led the Southeastern Conference in rushing last year with a school-record 1,391 yards.
"This puts no more pressure on me than there already was," Dixon said. "I knew they were going to call on me, and now it's time to step up. This is what I've been waiting for, and I'm definitely out to show some people what I'm capable of."