SANTA CLARA — Colin Kaepernick had a simple explanation for what made Michael Crabtree his favorite 49ers receiver last season: "He got open." Not so simple is figuring out how the 49ers will replace Crabtree for at least the first couple months this season.

Crabtree underwent surgery Wednesday at Stanford University to repair his right Achilles tendon, which he tore Tuesday while running a route on Day 1 of organized team activities.

"Obviously he's a great player and has done a lot for this team," Kaepernick said. "We just need to have someone step up."

The 49ers acquired proven veteran Anquan Boldin in a March trade with the Baltimore Ravens, but aside from him, the receiving corps is filled with question marks.

"We have a lot of talent out here, a lot of great players," Kaepernick said. "It's just: 'Who's going to step up and fill that role?'"

Potential starter Mario Manningham is recovering from a knee injury, having torn his anterior cruciate ligament Dec. 23. Kyle Williams is further along in his recovery from an ACL tear sustained on Nov. 25.

Younger options include A.J. Jenkins (2012 first-round draft pick), Marlon Moore (free agent from Miami Dolphins) Ricardo Lockette (2012 practice squad, Kaepernick's roommate), Chad Hall (2012 practice squad), Joe Hastings (2011 practice squad), Quinton Patton (2013 fourth-round pick) and Chuck Jacobs (undrafted rookie).

"The job we're competing for just happens to be one of the most important jobs on this team, and that's replacing the best receiver we had on the team," Lockette said. "We're not taking this lightly. Hopefully we can replace Crab and hold it down until he gets back."

Jenkins and Lockette spent mid-February through mid-April training with Kaepernick in Atlanta. Hall also worked out with them in Atlanta, and he doubled as a punt returner during Wednesday's practice.

"They're great players and they've been working hard," Kaepernick said of the duo.

Lockette said that training has been extremely beneficial, as has Kaepernick's off-field teachings about the 49ers' schemes.

Linebacker Patrick Willis was on the field when Crabtree went down after what Willis called a "simple, normal cut" during a 7-on-7 drill.

"Crab is one of those tough guys. He can get hard and he'll bounce right back up," Willis said. "Yesterday he stayed down a little longer than normal. You say a little prayer and hope it wasn't as bad as it looked. Unfortunately it is.

"At the same time, it's an opportunity for someone to step up and fill those shoes. Those are huge shoes to fill, but I know Crab is going to work hard to get back as soon as possible." Asked if he would like the 49ers to re-sign free agent Randy Moss, Kaepernick replied: "It's not my decision."

Kaepernick's focus instead is on improving his passing accuracy, knowledge of the playbook and, in an ever-more obvious need, "getting more comfortable with my receivers."

For more on the 49ers, see Cam Inman's Hot Read blog at blogs.mercurynews.com/49ers. Follow him on Twitter at twitter.com/CamInman.