"He went as long as he could," Jackson said.
Sources told ESPN's Adam Schefter the team is concerned Garrett could miss time, including Sunday's opener.
Garrett will undergo further evaluation Wednesday night. He was listed as limited on the injury report. Jackson would not guess whether this would affect the rookie's availability for Sunday's opener against the Pittsburgh Steelers.
"We'll see," Jackson said. "I don't want to make medical decisions. I'm not very good at them. We'll see where we are as we move through the week."
Garrett had a foot issue in minicamp, but Jackson said he did not believe the ankle injury was related. Garrett was the first overall pick in this year's draft.
"He was out there at practice; obviously he didn't finish practice," Jackson said. "Let's just leave it at that. He didn't finish today, so we'll see where we are."
When he met the media before practice, Garrett refused to back down from his statement in the spring that he hoped to take down Steelers quarterback Ben Roethlisberger.
"Stand by it," Garrett said. "You shouldn't be scared to take anybody down."
"I've been playing this game for 14 years now," Roethlisberger said Wednesday. "He's not the first or the last one to talk about trying to get me. I'm just blessed to have a group in front of me to help protect that. ... I know they are always up for the challenge. They always want to protect me, open up things in the run game. They take pride in being the best in the business and that's what I think they are."
As for Garrett's thoughts on trying to take down the 6-foot-5, 240-pound Roethlisberger after watching him on tape? "He is no small fella," Garrett said with a smile.