Posted by ESPN.com's Kevin Seifert
MANKATO, Minn. -- At this moment, I can't tell you whether Minnesota quarterback Tarvaris Jackson is seriously injured, dinged or just shaken up. We haven't gotten any word from Minnesota and might not until after the team's second practice of the day, which isn't scheduled to start until 8 p.m. ET.
Update (2:40 p.m. ET): Jackson has a sprained medial collateral ligament in his left knee, according to the Star Tribune.
No matter the degree, however, this episode should be a clear reminder about the dangers of counting on Jackson to stay healthy for 16 games this season. Aside from the debate about skills, Jackson has suffered a significant injury in all three of his previous NFL seasons. Even if he were to beat out Sage Rosenfels and win the Vikings' starting job this summer, his injury history doesn't inspire much confidence that he would stay on the field for the entire season.
Saturday morning's episode is the latest example. About 20 minutes remained in practice when Jackson stepped up in the pocket during a team passing drill. Players were in shorts and shoulder pads, and coaches had been reminding them repeatedly to stay off the ground at the end of plays.
Unfortunately for Jackson, however, defensive lineman Letroy Guion had slipped while locked up with center John Sullivan. Jackson didn't see them and fell awkwardly over Guion. Jackson rolled on the ground in pain for about a minute while athletic trainers and coaches attended to him.
Jackson ultimately walked off the field under his own power and hasn't been seen since.
I won't begin speculate on the severity of Jackson's injury other than to repeat that he walked off the field on his own. If nothing else, however, this event offers the Vikings reason for pause as they determine whether Jackson, Rosenfels or someone else (more on that in a bit) should be their starter.
Consider Jackson's injury history;
He tore cartilage in his knee during practice as a rookie in 2006 and had arthroscopic surgery.
A strained groin, a fractured finger and a concussion limited him to 12 games in 2007.
He sprained his right knee during the preseason last year, forcing him to miss the rest of summer practice. Jackson later said the injury played a part in his poor play early in the regular season.
Again, this injury might not be serious. But the fact that Jackson couldn't make it through the third practice of the summer without at least a hiccup is worrisome. And it was hard not to wonder what the Vikings would do if Jackson or Rosenfels does suffer a significant injury. Would they make the remaining quarterback their starter? Would they seek a new entrant to the competition? Could they possibly consider re-recruiting Brett Favre?
I'm not going there, not yet. But I'm betting I'm not the only person who is thinking that today.
I'm locked down here at Minnesota State University, Mankato, and will update you whenever possible.