Sumlin, who played linebacker at Purdue during the mid-1980s, served as OU's special teams and tight ends coach from 2003-05 before moving up to co-offensive coordinator and wide receivers coach—that's the same position Jay Norvell currently holds at OU.
From there, he accepted the head coaching position at Houston back in 2008 and was just named the Aggies' head coach in December 2011 following Mike Sherman's firing.
Just a year later, he's coaching against his old crony in a bit of an unexpected bowl matchup.
Stoops, meanwhile, is left looking across the sidelines against his 18th former assistant in a head coaching face off.
"Well, we're both professional," Stoops said. "Friendships and all that matter more. We can still be friends. Heck, Kevin and I can go out to eat two nights or the night before the game and still go out ready to beat each other. When you get on the field, you do what you do, you know, as Kevin would say."
Those words bring back to mind a story about Sumlin.
"He patented that phrase," Stoops said. "We were in our team review before one of the games, and it was at Tech, he got stumbling on his words and says, ‘It's a good day to go out.' He got kinda tied up. And he went ‘to play or have fun' or ‘to do what you do.' And that was how many years ago? You hear it a lot anymore. I'm sure he wasn't the first, but [it was] the first time we had ever heard it and we all were doing our best not to start laughing."
It was one of many good moments the two shared together.
Stoops wouldn't go into the best one, though.
"I can't speak about it to the media on the line," Stoops joked. "I think sharing the championships that we won together [was the most special]. We had a lot of good quality wins and championships together that we all, with our wives and kids, we always made sure we enjoyed ‘em."
Sumlin admitted those, indeed, remain some of his fondest memories during his five years as a Sooner assistant with Bob.
"There's a bunch of great wins, a bunch of great games," Sumlin said. "The Big 12 Championship game in Kansas City against Nebraska, that was a great moment. [There were] a lot of great wins. I think the culture that he created, not only for the team, but as he said, for our families, because our families, our wives are still close. It's a complete family culture.
"As I said, it's not just one single experience in five years; there's a bunch of things that go into a great experience like I had at Oklahoma."
And no matter how many memories the two had on the field together while coaching at Oklahoma Memorial Stadium, which included three Big 12 Championships in five seasons, 54 total wins and four BCS bowl game appearances, two of which were national championship games, the most lasting thing might be their aforementioned friendship.
"It's really not different," Stoops said of when they get on the field. "I mean Kevin and I are great friends. We just—our wives are great friends. We've spent time in New York just a week or two ago. But in the end once you get to playing, you're playing. You know, and bowl games are a positive thing, so you're in that situation.
"You're both in good shape, you know, so it's just something you gotta go do. So, it's really not all that different, you know."
The two will certainly put their friendship aside when their collective teams meet Jan. 4 in Arlington, Texas.
But you can bet as soon as it's over they'll be back to being buddies once again.