Just two games into the preseason, that question has already been raised again.
Jackson sprained his right knee in the first quarter against Baltimore on Saturday and his status for next Saturday's game against Pittsburgh is in question. The quarterback participated in individual drills and did some throwing at practice on Monday, but did not do any team work, coach Brad Childress said.
"Guys don't always feel chipper in the first part of the week when they get dinged up, but they find a way to push to Saturday," Childress said. "That's how he's approaching the thing and that's how we're approaching the thing -- to play in Saturday's game."
The Vikings would appear to be in a tough position this week. The safe route would be to give Jackson the night off and let veteran backup Gus Frerotte start. But the third game is also the most important of the preseason because the starters see significant playing time before taking it easy in the finale to avoid injuries.
Jackson has looked sharp in his first two preseason games, going 15-for-22 for 200 yards and two touchdowns in about five series. But he also is entering just his second full season as the No. 1 quarterback, and he needs all the snaps he can get as the regular season approaches.
Jackson was unavailable for comment.
"I'm sure it's his wish to get as many turns as he can," Childress said.
Throughout training camp Jackson has been bombarded with questions about his readiness to lead a veteran team and his comfort in running Childress' West Coast system.
A side plot to that uncertainty is Jackson's ability to take the field for all 16 games. He missed four starts last season because of injuries, but went 8-4 in games he did start.
He spoke openly of his frustration with getting injured last season and said it was the first time in his football career that he had difficulty staying healthy.
That's common for young players in the NFL, Childress said.
"Numbers of them haven't dealt with anything significant before," he said. "Sometimes they just flat don't know what to do. 'Do I shut down and sit down? Do I show up and treat? How do you do that?'
"They have never been part of a training room culture, nor do they want to be."
Childress lauded Jackson for coming into training camp in great shape and his willingness to work with athletic trainer Eric Sugarman when rehab is necessary. If he can't go on Saturday, the 37-year-old Frerotte will take his place.
"I'm sure he's going to be fine," Frerotte said Monday. "Injuries happen. It's a part of football. He'll work hard to get back out, and he'll know when he's ready."
The Vikings signed Frerotte for this purpose. When Jackson was injured last season, they had to resort to veterans Kelly Holcomb and Brooks Bollinger, neither of whom proved to be reliable running the offense.
Now it could be the strong-armed Frerotte's turn to show he is an upgrade. He went 4-for-9 for 32 yards and a touchdown against the Ravens.
One major difference between the two is that Jackson is much more mobile, while Frerotte is the more classic drop-back passer.
Jackson scrambled twice for first downs against the Ravens, adding a dimension that demands the attention of the defense. But he was also injured when he didn't slide, which is something that no doubt will be addressed as Jackson returns to action.
"I think he understands the fact that he needs to get out there for 16 games this year," Childress said. "Maybe we learn to live to fight another day and take 2 less yards up the field."