Guerendo was visibly frustrated after walking to the sidelines, hoping the injury wasn't serious. Several of the team's other running backs, including fullback Kyle Juszczyk, came over to check on and console their first-year teammate.
Guerendo did not return to practice.
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Following the Wednesday morning session, head coach Kyle Shanahan informed reporters that Guerendo is dealing with a hamstring injury.
"It looked like he hyperextended a little bit," Shanahan stated. "He was going about half-speed, it looked like, so it was an unusual one. They just told me it was a hamstring, but we got to get the MRI."
It's a tough situation for Guerendo, who joins a crowded and talented running back room that features Christian McCaffrey, Elijah Mitchell, and Jordan Mason. Guerendo will need practice reps to climb the depth chart.
San Francisco is excited about Guerendo's potential. His 4.33-second 40-yard dash at the NFL Scouting Combine was the fastest among this year's rookie running backs and the third-fastest overall in Indianapolis. It's Guerendo's speed that caught the 49ers' attention during the pre-draft evaluation process.
"Number one, we were looking for speed," running backs coach Bobby Turner said in May. "And not only speed, but he has the speed, and he's bigger. He's 221, 222 pounds. So we needed another bash runner in our offense. The head coach and everyone else can speak for themselves, etcetera, but he doesn't have a lot of wear and tear on his body, but he's very coachable.
"... But the very first thing I saw on tape was the ability to break the long runs and take it to the house. He's gonna hit the home run."
The team remains hopeful that the MRI results will show the injury is minor, allowing him to return to practice soon and make an impact.