Graunke threw for three touchdowns in the third quarter and ran for another score as No. 16 Hawaii defeated Utah State 52-37 on Saturday night.
The victory gave the Warriors (6-0, 3-0 Western Athletic Conference) a school-record 10 straight conference wins and their best start since the 1981 team opened 7-0.
"I felt real comfortable," Graunke said. "I didn't get a chance to warm up or anything but I took over where Colt left off."
Brennan walked off the field on his own power after taking a hard hit with 52 seconds left in the first half with Hawaii leading 24-14. The Heisman Trophy hopeful completed a short pass when he was drilled by linebacker Paul Igboeli.
Recovering from a right ankle injury he suffered three weeks ago, Brennan laid on the field for a few minutes before slowly getting up and gingerly walking off the field.
He was replaced by Graunke who completed a 38-yard pass on his first pass to set up his 1-yard TD run that put Hawaii up 31-13. Graunke was initially ruled down with 5 seconds left in the half, but the call was overturned after a review.
Brennan, ranked second in the nation in passing, returned in the third quarter to a round of cheers and played one series before limping off the field for good. He was 19-of-25 for 219 yards and one touchdown.
Brennan, coming off a career-high five interceptions performance in last week's 48-20 victory over Idaho, said he tried returning with an ankle brace, but it just aggravated the injury as his ankle swelled up.
"It's been frustrating the last couple of weeks -- feeling not right. [But] we're 6-0 and we're rolling and I'm fine with that," he said.
Graunke completed his first nine passes for 246 yards and finished the night 9-of-11. He connected with Jason Rivers on a 72-yard pass play to put Hawaii up 52-27. Rivers had seven catches for 130 yards, moving him into second place all-time in receiving yards at Hawaii, behind Ashley Lelie.
"I always known that he could play," Warriors coach June Jones said of Graunke. "I'm proud of him and happy he had this success."
"Don't give me any credit. Tyler came in and played amazing. It wasn't me," said Hawthorne, who led Hawaii with nine catches for 116 yards.
The Warriors, surprisingly, had a difficult time with the winless Aggies (0-6, 0-2), who extended their losing streak to 12 games.
"Teams like this are hard to beat," Hawaii linebacker Solomon Elimimian said. "They have nothing to lose coming into this game. I think we may have underestimated how much they wanted it."
Utah State, which hasn't won on the road since 2005 and dropped to 1-43 against ranked teams, was led by the play of Kevin Robinson.
Robinson returned a kickoff 100 yards for a touchdown and caught five passes for 96 yards, including a 77-yard TD catch that cut Hawaii's lead to 25.
Robinson's dazzling kickoff return gave the Aggies a 10-7 lead. The speedy Robinson caught the ball at the goal line and zigzagged his way until he found daylight near midfield. He ran untouched the rest of the way, bring to life the Aggies sideline while stunning the home crowd.
Utah State, which entered the game averaging just 15 points a game, kept Hawaii off balance with its mobile quarterback Leon Jackson III, who had 161 yards passing and 55 yards rushing.
The Aggies played confident and physical. They clearly were not intimidated by the Warriors.
Utah State coach Brent Guy said his team would be playing well, "but when something big happens to us, we disintegrate."
"You got to score touchdowns to play with Hawaii, you can't kick field goals," he said. "They had about eight big plays that killed us."
Derrvin Speight's 9-yard TD run cut Hawaii's lead to 11 early in the third quarter, but that's as close as the Aggies would get as Graunke took over.
Peter Caldwell's 36-yard field goal early in the second quarter gave Utah State a 13-10 lead that it would hold for just 12 seconds. Malcolm Lane scored on an 87-yard kickoff return, giving Hawaii a 17-13 lead.