"It is a torn ACL, just a straight ACL," Hendrickson said during a KNBR interview. "Nothing more to it than that. Nothing else involved. Just a torn ACL."
Hurd will undergo surgery in "a couple of weeks," said Hendrickson.
The prognosis was what head coach Kyle Shanahan expected.
"I saw that the ACL was reported today. It would be a surprise if it's not," Shanahan told reporters after Monday's practice. "It happened yesterday. He's having the MRI right now, but it's a pretty unfortunate thing for him ... For him to get hurt before he actually got to go out there, just a freak thing on air, cutting."
Hurd will land on injured reserve, Shanahan said later during the press conference.
"I really felt bad for the guy, but he'll be back next year," the coach added.
The 49ers had been trying to ease Hurd back after the 6-foot-5 receiver missed his entire rookie season with a back injury. He was limited during the team's first two practices of training camp, participating in individual drills, and then moving to a side field for some more individual work during team drills.
Initial reports from those who observed the injury were that it didn't appear to be too serious. Hurd spent a few minutes down on the field but was able to walk off, albeit gingerly, to the locker room.
The injury was obviously more severe than most thought.
"Jalen knows full-well that he needs to work his tail off so he can get back and be a big contributor, and try to earn opportunities to be a contributor for our team moving forward," general manager John Lynch said in April. "We love his skill set. We love his mindset. Now, it's time to put it all to work."
The 49ers made Hurd a third-round draft pick out of Baylor in 2019.
Mike Garafolo of NFL Network reported on Monday that the 49ers plan to work out veteran wide receiver Jaron Brown this week. He'll need to pass three COVID-19 tests before that can happen.
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