NFL Network's Jason La Canfora reports that Jackson's MRI revealed a mild, Grade 1 MCL sprain, according to a league source. The Vikings quarterback is expected to return to practice some time next week.
Jackson was hurt when defensive lineman LeTroy Guion fell at his feet and rolled over his leg during an 11-on-11 drill.
Jackson folded to the field in pain as trainers rushed to check on him. After a few minutes, he was able to get up and walk off under his own power.
"I really don't know what happened," Guion said. "Everybody was moving so fast."
The fourth-year pro first received some attention at a tent on the side of the field before heading to the team's locker room area at Minnesota State University, Mankato.
"Whether it's Tarvaris or any of your guys, you don't want to see any of your guys go down, no matter who it is," offensive coordinator Darrell Bevell said after practice. "Whether it's your quarterback, your third tight end, it doesn't matter. You don't like to see anybody have any type of injury."
The Vikings have held three practices since training camp began on Friday, and Jackson and Rosenfels have been splitting the snaps with the first team.
"It's pretty tough (to see that), especially in practice," left tackle Bryant McKinnie said. "Hopefully he'll be all right."
In his first season on the job in 2006, Vikings coach Brad Childress drafted Jackson in the second round as his quarterback of the future. He has been up and down in his first three seasons in the league, which led to the recruitment of Favre all summer long.
When Favre called Childress on Tuesday to tell him he wasn't coming, the competition started anew for Jackson and Rosenfels.
Jackson's familiarity with the offense appears to have given him a slight edge so far, but it's still very early in the competition.
"At this time, Tarvaris has a little bit more knowledge of our system," Bevell said. "He is an athletic quarterback. He's got a very strong arm and those would be the positives about him."
Information from the Associated Press was included in this report.