Replays showed Jackson's helmet colliding with that of offensive tackle Cedric Ogbuehi as the two engaged at the line of scrimmage. Jackson fell to the turf and remained there for several minutes before the team's medical staff strapped him onto a backboard, with the face mask removed from his helmet, and loaded him into an ambulance.
Jackson regained consciousness and has feeling in his arms and legs, according to a source, and was taken to the hospital and released Saturday night.
He is wearing a neck brace as a precaution and will continue to be evaluated, a source told ESPN.
"I didn't catch the replay, but he hit heads, I think, with Cedric and it might have caught him in the chin with his helmet, and it was like a knockout punch is what it was," Carroll said. "So we took all the precautions and took care of him. He had movement and all that kind of stuff, but we still needed to do the right thing and totally take care of him until we know more."
The play occurred just before what would have been halftime of the Seahawks' first scrimmage of training camp at CenturyLink Field. Carroll said they ended the scrimmage early out of respect for Jackson, calling him "one of our all-time guys, favorite guys."
"I didn't think it was right to try to get everybody cranked back up again and go back out," he said. "We didn't need to do that, but we had gotten enough done. We'll take the plays that we got and what we learned from it. We accomplished the things we needed to accomplish coming to the stadium."
Jackson, 27, is entering his fifth NFL season. He went undrafted out of Texas Tech in 2016, spending his rookie season with the Raiders and the past three with Seattle. He has been competing for a roster spot as one of the Seahawks' rotational defensive ends.
"I'm praying that he'll be OK," quarterback Russell Wilson said, adding, "I believe he'll be OK, it sounds like."
According to stats provided by the Seahawks, Wilson went 8-of-13 with a touchdown in his two drives, both coming against Seattle's No. 2 defense. His touchdown pass was to wide receiver DK Metcalf.
The Seahawks have two more scrimmages scheduled over the next eight days.
"We didn't get a whole bunch of plays today, but we got sequences and we got situations and we got enough third downs to challenge Russ and the offense over there, so there was some really good things that happened," Carroll said. "Everything went really well other than that we got BJ knocked out. That was terribly unfortunate."