Erin Henderson admits there was at least an instant of intense worry when he had the back of his left foot stepped on during Thursday's practice. The middle linebacker's ankle was cut open in the back near his Achilles tendon and the initial sensation was powerful.

"Really, really painful," Henderson said. "I've been playing ball since I was 7 years old and I've never had anything like that happen to me before. … It was on fire. There was a little bit of fire down there. But once it calmed down a little bit, then I realized it wasn't too bad."

Henderson didn't practice Friday as the Vikings finished up preparation for this weekend's preseason game in San Francisco. But the starting middle linebacker will be on the field and at full-go Sunday at Candlestick Park.

"Nothing to be worried about," Henderson said.

Audie Cole continued to take reps with the first-unit Friday in Henderson's place. But the defense should be close to full strength for the first half against the 49ers. Henderson said he's looking forward to the challenge of facing San Francisco's read-option offense after two previous preseason tests against Houston's zone rushing attack and Buffalo's no-huddle, up-tempo packages.

"We've gotten a whole array thrown at us this preseason," Henderson said. "I think it's really good for us as a defense to be able to get some of these different looks and figure out where problem spots might be for us and what we need to work on and what we have to improve upon in order to be ready for the season."

As for Thursday's injury, Henderson also acknowledged he may have dodged two serious bullets. Just a moment after his ankle was injured, he stumbled across the sideline and kicked a cooler out of frustration.

"I think the cooler might have won that battle," he joked. "Actually, I looked down just in time and saw that there was a wet towel. So I assumed it was kind of full so I kind of backed off when I kicked in."