Next week’s “Monday Night Football” showcase could feature a quarterback matchup of journeyman Jason Campbell of the Bears vs. untested Colin Kaepernick of the 49ers. Not exactly the creme de la creme of the NFC.

Why? Concussions, of course.

With so much attention paid to replacement refs and bounty ruling appeals this season, concussions are an issue that’s slipped a bit under the radar lately. But it’s hard to ignore this: 25 percent of Sunday’s NFL games had a starting QB leave with a concussion.

Two were Chicago’s Jay Cutler and San Francisco’s Alex Smith, whose teams play each other next Monday. Both stayed in Sunday’s games for several plays after what appeared to be head-rattling hits. Smith even threw a TD pass while playing with blurred vision before he departed, according to Coach Jim Harbaugh.

“It’s a reminder that you’ve got to err on the side of caution,” said Dr. Richard Ellenbogen, co-chairman of the NFL’s head, neck and spine committee, who was not familiar with the particulars of Sunday’s quarterback injuries. “The question that I would ask is: Why did Mr. Smith not report this to his team physician, and say, ‘Hey, I’ve got blurred vision, is that a problem?’ … We have to educate the medical teams to be really conservative. And we still have to educate players to self-report. If they don’t feel 100 percent, they have to be willing to very strongly tell somebody.”

More than 3,500 former players — including at least 26 members of the Pro Football Hall of Fame — have sued the NFL, saying not enough was done to inform them about the dangers of concussions in the past, and not enough is being done today to take care of them.

The instructions now used for in-game sideline concussion assessments in the NFL include a box that reads: “Signs and symptoms of concussion may be delayed, and therefore it may be prudent to remove an athlete from play, not leave them alone, and serially monitor them over a period of time.” After that, in all capital letters, it reads: “When in doubt, take a ‘time out.”‘

The NFL looked into the Cutler and Smith cases — and Philadelphia’s Michael Vick, the third quarterback who got a concussion this weekend — and came away satisfied that the proper protocol was followed. Players who exhibit any concussion symptoms are supposed to be removed from a game immediately and not be allowed to return to play or practice until fully without symptoms.

“Our medical advisors routinely review with team medical staffs all significant injuries,” league spokesman Greg Aiello wrote in an email. “In these cases, we learned that the teams handled the injuries properly and removed the players from the game as soon as they displayed symptoms and were diagnosed with a concussion.”

Ellenbogen pointed out that concussion symptoms might take time to emerge. That’s apparently what happened with another noteworthy player, Buffalo Bills running back Fred Jackson, who took a late hit to the head in a loss at the New England Patriots on Sunday.

He was examined Monday — a day after showing what Coach Chan Gailey called “Concussion-like symptoms” on the flight home. Gailey said Jackson will miss Thursday’s game against Miami.

Bears Coach Lovie Smith said Cutler showed no symptoms of a concussion immediately after a helmet-to-helmet hit from Houston’s Tim Dobbins in the second quarter. Cutler finished the half, then was held out after halftime. While the play with Dobbins’ hit was being reviewed by officials, Smith said: “Our trainers talked to him, evaluated him; he was fine from there. Players in the huddle didn’t see anything wrong with him at the time.”

Vick, meanwhile, was diagnosed with a “pretty significant” concussion, Coach Andy Reid said. Reid wouldn’t rule Vick out for Sunday’s game at the Washington Redskins.