The Calgary Stampeder halfback left the game in the second quarter and did not return in an 18-13 loss to the Winnipeg Blue Bombers. On a nine-yard run, Cornish was upended by Blue Bomber defensive back Bruce Johnson.
Cornish's right shoulder hit the turf first and his head snapped down on the ground. The CFL's Most Outstanding Player in 2013 needed assistance getting off the field. Cornish walked to the team clubhouse unaided, but flanked by medical staff.
The 30-year-old from New Westminster, B.C., missed six games in July and August with concussion symptoms. Cornish was knocked unconscious by Montreal's Kyries Hebert in the season-opener.
Cornish told reporters in the Stampeder dressing room following Saturday's game he didn't think his second major knock to his head this season was as serious as the first on June 28.
"I knew I was concussed after that," Cornish said. "I was not at the same level I'm at now.
"I guess I got upended, another guy going for my knees as always and my head contacted the ground on the right side," he continued.
"When you hit it on the side, that's when you get the balance issue, so when I was getting up, I didn't have much balance so then immediately everybody freaks out.
"Go in, get checked out, no real issues, but as a precautionary measure, they took me out. Like I learned before with concussions, you can't say anything until you've got a few days without symptoms."
Calgary (14-3) closes out the regular season Friday in Vancouver against the B.C. Lions. Whether Cornish will play in the finale is a question mark.
The Stampeders secured first in the West Division with a win in Winnipeg on Oct. 18, and thus knew over a month out from the division final they'd be playing in it Nov. 23 at McMahon.
Cornish posted a single-game career-high 208 yards in Winnipeg and was working another big-yardage game Saturday before he was hurt. He rushed for 105 yards to surpass 1,000 for a third straight season before leaving the game.
He was appearing in just his ninth game after sitting out two other games this season with injuries. Cornish still leads the CFL in rushing with 1,082 yards after winning the league's rushing title the past two seasons.
Stampeder head coach and general manager John Hufnagel was optimistic Cornish will bounce back.
"I don't believe it's very serious," Hufnagel said. "He's on the sideline 10 minutes after he went in (to the clubhouse). He'll go through concussion protocol, but I don't think it will take very long."
New turf was installed at McMahon Stadium prior to this season, to the relief of the Stampeders who said the previous playing surface felt like concrete in recent years.
But Stampeder slotback Nik Lewis said the new turf lost cushioning in September, when snow removal after a freak summer snowstorm also removed the small black pellets embedded in it.
"We've got to figure out a way to replenish the little black pebbles or you're just going to play on concrete and everybody is susceptible to injuries no matter who you are," the veteran receiver said. "The pebbles are what makes it soft. It's like concrete if you don't have the pebbles.
"I'm just saying it's scary to see Jon go down with that head injury and everybody is susceptible to that because it's just a normal routine play. He hit his head on the ground and you see how hard the ground is."