Withrow, the club's first-round pick in the 2007 Draft, suffered a herniated disk in his lower back during October's instructional league and until this week had been limited to core-strengthening exercises.
Guerra, the closer on new manager Don Mattingly's Arizona Fall League club, suffered a gash on the top of his pitching hand in a dishwashing mishap near the end of the AFL season in November and is only now starting to toss.
Both, however, expressed confidence they would be ready for Spring Training, which starts for Guerra in a month (he's on the Major League roster) and for Withrow two weeks later when the Minor Leaguers report.
A bad back is the last thing Withrow needs. After signing for a $1.35 million bonus, he overcame a case of the yips and a sore arm in 2008, when extreme wildness limited him to only four innings. Demons apparently conquered, he had a solid 2009 season, mostly at Class A Inland Empire, then a six-game promotion to Double-A Chattanooga.
Withrow spent the entire 2010 season at Chattanooga with mixed results, going 4-9 with a 5.97 ERA. Although he's still only 21, more was hoped for from the right-hander.
Guerra made an impression on Mattingly in Arizona, only to need repair work on the area of the knuckle and tendon at the base of the middle finger. He said he was injured doing the dishes in soapy water, but wasn't sure what exactly did the damage. He was 2-0 with a 2.33 ERA at Chattanooga last year, despite 22 walks in 27 innings, but he also missed several months with tendinitis in his throwing arm.
Here are Mattingly's reviews of several other Dodgers prospects that played for him in Arizona:
Ivan DeJesus Jr. -- "He swung the bat well. I know since the injury [a broken lower leg] they say he's a step slower here or there, but he's prepared to hit at the Major League level. He has a good feel for how pitchers try to handle him. I thought he was really good offensively. Defensively, I didn't see him as much as I'd like to because of the restrictions on who plays where. Offensively, the kid is ready to hit and produce. Where he fits, who knows?"
Jerry Sands -- "He was impressive. I loved his swagger. You hear that a lot, and how to explain it, for me it's when a guy gets him out and he walks back with a look like, 'OK, you got me this time, but I'll be back.' There's a confidence about him. He's interesting. He plays the outfield really well. Without putting a tag on him that's not fair, he kind of has a Matt Holliday big, strong body. He runs better than it looks and he uses the whole field and he's confident."
Trayvon Robinson -- "He's coming. I've had the chance to see him play with my son in Michigan, saw him in the spring and fall, and every time I see him, I love to see the progression. Each time he's gotten better. He's getting there. If he continues to progress, he's got a chance to be an impact guy."
Scott Elbert -- "He was tremendous for me. His stuff plays. I've talked to him. He's been up and down, up and down. He's got a power arm and can get lefties or righties out. He seems durable. He's not afraid. I like him."
Steven Ames -- "His numbers weren't good, but he threw the ball good. He had a couple bad breaks or he could have been out of innings."
Ken Gurnick is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.