He told the 6-2, 300-pounder that he had not fully taken advantage of his athletic ability during his college career. Gundy then promised that in the bowl game he would play Castleman on offense and that he would let him carry the ball.
That promise was more than kept in the 30-22 Cactus Bowl victory over Washington. The promise kept to the senior will also go a long way in promoting promise for the future.
"I felt bad at the end of this season and I looked at him and he such athletic ability and he has great hands," Gundy said in the Cowboys locker room after the Cactus Bowl. "He has freakish athletic ability for a 300-pounder. I thought that I ought to be hit in the head with a board for not using him on offense.
"I was so happy when we won that game (OU) that the next day I made that announcement, and I hadn't even told the offensive coaches yet. I said, 'we're going to use Castleman and let him have some fun,' and not only did he have fun but we should have been using him for four years. It is what it is, he kicked out and blew those holes open and then he reaches out and catches it like he's been doing it all his life."
He sure did, as the new heavy package on the Oklahoma State offense, which also included fellow defensive tackle Ofa Hautau, featured the big defenders blocking and Castleman as a ball carrier, which he was powering over from the 1-yard line for the first touchdown.
Then he made an impact even as a receiver, although he claims he was not the primary receiver on a 48-yard catch and run in the fourth quarter to help put the game away.
"Honestly, I didn't think he was going to throw the ball to me until the play was going and the guy that was supposed to be guarding me fell," Castleman said of the pass play. "I was like, 'Oh man, Mason's going to throw me the ball. Oh gosh, what's going to happen?'
He threw it to me, concentrate, concentrate, catch. Took off and I started running and they hit me. I didn't go down so I kept running and I was like nobody has hit me yet and kept looking to see where they were at. I glanced up at the screen (video board) and I thought, 'Oh gosh, they're right there.' I tried cutting back in and that wasn't happening and the check engine light came on and I had to go down."
It was a great play to watch and Castleman can now count that promise of using his athletic ability made when Gundy was in his home recruiting him as a senior out of Amarillo High School and renewed at the OSU Senior Banquet as absolutely kept.
"I'm happy I was able to do it," Castleman said. "Help out the team and make the fans proud."
Gundy said, "I was glad that I finally got that done and it would have been a big regret if I hadn't put the ball in his hands."
Now that promise kept leads to an open invitation for ball carriers by trade, not just big athletes that play elsewhere and are capable, to jump in and shine in the Oklahoma State offense.
The fans are in an uproar in recruiting because the Cowboys had a premier running back in McKinney (North), Texas All-American Ronald Jones II committed for eight months. The rumor was USC recruiters and fans pointed to Oklahoma State and their struggle to run the ball at times behind a very young offensive line and convinced him that the other "Tailback U" was where he should go.
Now all of a sudden the young offensive line is coming around and the Cowboys have recruited several really promising junior college linemen and had a transfer in starting left tackle Victor Salako from UAB.
"You had a freshman and a freshman, we had a sophomore, a senior, and a sophomore starting for us on the offensive line against a really good defensive group up front for Washington and we ran and passed the ball," Gundy said about the offense line in the Cactus Bowl.
With starter Desmond Roland graduating and speedster Tyreek Hill no longer on the squad, there is lots of playing time including the opportunity to start right away. Add to that a very promising young quarterback and overall receivers in place that should make running the football an easier proposition.
"We have to get on the recruiting trail," Gundy said. "We have to continue to develop who are and we've got to keep upgrading what we do."
The Cowboys have just about everything left in place as they will return eight offensive starters from the Cactus Bowl and seven starters on defense. Add to that nine other players returning that started one or more games on offense this season and another three that started a game or more on defense in 2014.
The Cowboys have a lot of experience coming back and a lot of it is very young experience. The one main area of need is running back where only Rennie Childs and redshirt freshman Sione Palelei are back.
OSU has offered a number of running backs and in this day of de-commitments of which the Cowboys lost Jones then it could be some backs see how attractive the situation is in Stillwater that they make a second choice that looks better than their first.
It sure looks inviting to jump into a situation with lots of playing time available where guys like Barry Sanders, Thurman Thomas, Kendall Hunter and Joseph Randle have stepped from Cowboys success to NFL success.
It's time for Gundy and his staff to sell the OSU program, and thanks to Castleman and the promise kept it ought to be very easy for 180- to 200-pound speedsters to see how well they can do in orange and black with an imaginative offensive staff and behind a young and now coming of age offensive line.