It was another sudden blow for the 49ers, who saw defensive lineman Kentwan Balmer leave the team Monday. Balmer, a first-round draft pick in 2008, has been unexcused since Wednesday, and he faces tough odds to make the team even if he does return, team sources told NFL Network insider Jason La Canfora on Friday.
"We just respond to this and move forward," 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." However, a league source told La Canfora that Coffee hasn't yet filed retirement papers with the NFL, and some are skeptical about him doing so.
The 49ers could try to recoup part of Coffee's signing bonus should he retire, according to La Canfora.
Coffee, a third-round draft pick in 2009, was the 49ers' second-leading rusher last season with 226 yards, but he averaged just 2.7 yards per carry and scored only one touchdown.
Singletary, who said Coffee "was having a fine camp," admitted that the running back's abrupt decision caught the team by surprise. Coffee didn't show up for Thursday's afternoon practice, 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."
Singletary said he didn't try to talk Coffee out of leaving the team.
"My initial reaction was, 'Glen, are you sure about this?'" Singletary said. "He said, 'Yes, sir, I've given it a lot of thought,' and I said, 'OK. If that's where you are, then I respect that, and I appreciate your honesty. I appreciate the character that you have to come and let us know so that we can move forward and not put us in a situation where we have to go scramble and do something that we didn't want to do.'"
Robinson averaged 4.7 yards per carry as a backup to Gore in 2007, but he seldom has 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 gradually brought Dixon along, but his learning curve now will accelerate. Dixon will be Gore's top backup in Sunday's preseason opener at Indianapolis.
"It's one of those things where he is going to take more snaps; he's going to get more of the workload," Singletary said of Dixon. "He's really going to have to step it up in terms of his learning progression and his learning of things."
Dixon has been eager for 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."
The Associated Press contributed to this report.