(This article has been updated with new information.)
NORMAN -- There was plenty of energy around Owen Field for OU's SEC opener against Josh Heupel and Tennessee.
The offense, already a question mark going into the game, turned the ball over three times -- all by Jackson Arnold, two on fumbles and one interception -- as No. 7 Tennessee beat No. 13 OU 25-15 on Saturday night.
The Sooners managed just 222 yards of total offense -- 124 of those coming in the fourth quarter.
Here are five takeaways from the loss, including trouble at quarterback for the Sooners, concerning injuries and Heupel's return:
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Late in a disastrous second quarter -- more on that in a bit -- Sooners coach Brent Venables and offensive coordinator Seth Littrell pulled the trigger on a change.
Freshman Michael Hawkins Jr. came on to replace a healthy Jackson Arnold.
Arnold was 7 of 16 for 54 yards and an interception and was officially credited with five carries for minus-21 yards.
Hawkins was thrown right into the fire, as the first snap he saw, from center Branson Hickman, was well wide.
Hawkins was able to recover the muff to keep the possession with the Sooners, but OU once again had to punt.
The Sooners punted on the first four drives with Hawkins at the helm, though the freshman did show flashes.
Then OU's offense finally showed some life early in the fourth quarter, with Hawkins leading an 10-play, 68-yard touchdown drive.
The freshman appeared to finish it off with a tough 12-yard touchdown run, fighting through multiple defenders at the goal line to get into the end zone.
But the play was overturned on review, giving the Sooners the ball six inches short of the goal line.
Two plays later, Hawkins hit Jovantae Barnes for a touchdown.
But OU's Jacob Sexton was penalized on what looked to be a successful two-point conversion, and then Tyler Keltner missed the 35-yard extra point.
Hawkins finished 11 of 18 for 132 yards and a touchdown and had 12 carries for 22 yards.
Arnold returned for one play late in the fourth after Hawkins was shaken up on an apparent touchdown -- that was later overturned -- but after Jovantae Barnes scored, Hawkins came back on for the two-point conversion attempt.
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Twice in the second quarter, OU's defense came up with a big-time play to put the Sooners in prime scoring position.
The Sooners' rattled Nico Iamaleava for two fumbles.
Each time, though, OU's offense coughed the ball right back up.
Early in the quarter, Robert Spears-Jennings blasted Iamaleava from the blind side and recovered at the Tennessee 6 by Ethan Downs, who brought it one yard closer.
But the Sooners' perfect red-zone scoring record came to a quick end.
On the first play of the ensuing drive, Jackson Arnold kept the ball and was hit in the backfield by Tennessee's Joshua Josephs. Arnold couldn't hold onto the ball and Josephs came up holding the ball high.
Arnold tried to convince officials he was down before losing the ball, but a replay confirmed the fumble call.
Later in the quarter, with OU trailing 12-3, Trace Ford forced an Iamaleava fumble and Gracen Halton recovered at the Tennessee 33.
But once again, disaster immediately struck.
The Volunteers brought pressure up the middle and Arnold had to quickly throw it out to freshman Zion Ragins near the line of scrimmage.
Ragins dove for the ball but couldn't haul it in, and the ball was sent bounding down the field, where it was eventually recovered by Jakobe Thomas.
This time, Tennessee took advantage, with running back Dylan Sampson carrying the ball on eight consecutive plays, scoring on a 1-yard run to put the Vols' up 19-3 at the break.
OU's second quarter was catastrophic.
In addition to the two turnovers, the Sooners had minus-20 yards in the quarter, minus-18 rushing, Jovantae Barnes was brought down for a safety and the Sooners were 0-3 on third down. OU had the ball for just 3:13 in the quarter as the Sooners did not pick up a first down.
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While OU's offense was ... less than stellar, the Sooners' defense mostly held up well against a Tennessee offense that came into the game posting monster numbers.
OU forced two Tennessee turnovers and mostly held Vols' young star Nico Iamaleava in check.
But the performance wasn't without its issues defensively.
Iamaleava completed four passes of 38 yards or more, with 196 of Iamaveava's passing yards coming on four completions.
His other nine passes went for a combined minus-4 yards.
Tennessee finished with 345 yards of total offense, including 151 on the ground.
Robert Spears-Jennings finished with 13 tackles while Danny Stutsman, the Sooners' leading tackler, had just three.
The Volunteers had punted just thrice over the first three games but punted eight times Saturday.
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OU was hoping Nic Anderson's return would help the passing game get back at least heading in the right direction after three lackluster passing games.
But Anderson quickly was sidelines again.
The redshirt sophomore subbed himself out on a third-down play and went back to the locker room without trainers.
Anderson later emerged in street clothes.
He did not record a catch in the game, his first appearance of the season.
Anderson had 31 catches for 798 yards and 10 touchdowns last season.
The Sooners had another costly injury about six minutes into the second quarter when starting cheetah Kendel Dolby, who missed OU's win over Tulane with an injury, suffered a left leg injury.
Dolby had three tackles in the game, a strong performance in his return.
After Dolby was loaded up onto a medical cart on the field, the Sooners' sideline emptied as players, coaches and staff went over to offer support for Dolby, who was seen covering his face as the cart took him to the locker room.
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Less than 30 minutes before kickoff Saturday, Tennessee coach Josh Heupel was honored at Gaylord Family -- Oklahoma Memorial Stadium.
"Welcome Back Josh Heupel," the video board said, along with a picture of Heupel after leading the Sooners to a national championship win over Florida State to end the 2000 season.
Alongside was live video of Heupel during warmups.
Heupel tapped near his heart as he was acknowledged on the public address system and received a strong ovation from the crowd.
Heupel starred for the Sooners in 1999-2000 and began his coaching career on Bob Stoops' staff.
But in 2014, with Heupel as co-offensive coordinator and quarterbacks coach, the Sooners' offense struggled and Stoops let Heupel and Jay Norvell go after the season.
Heupel rebuilt his career with stops at Utah State and Missouri as offensive coordinator before getting his first head coaching job at UCF in 2018.