"He's a kid... in the locker after the game, [I told him] he's the only guy that's got some fire right now," said O'Brien. "I said to him, ‘Thank God you're here, man. You're my kind of guy.'"
Thornton's emergence this year has been a blessing for NC State. The Wolfpack opened the year planning to redshirt Thornton as he was fifth on the depth chart behind Mustafa Greene, James Washington, Tony Creecy, and Brandon Barnes.
However, Greene was kicked off the team, Washington missed nearly the entire year with various injuries, and Creecy has been banged up for much of the season, so just three games in Thornton would play for the first time against The Citadel.
He made the most of it, rushing for 145 yards and two touchdowns on 21 carries in the 52-14 win. He followed it up with 101 total yards versus Miami and a big touchdown against Florida State before going three games without rushing for over 28 yards.
He got back on track two weeks ago with a 27-carry, 110-yard performance in the 37-6 win over Wake Forest and he tallied 114 yards on 21 carries and a score against Clemson with another 53 receiving yards.
"He hit the wall there for a while," said O'Brien. "Right now he's healthy and we need him. He's had two really good games.
"It's good for this football team that we've established a running back like that. He runs hard, he runs with power, he's got good vision, he's tough to bring down. He's starting to feel much more confident in where he is going and what he is doing."
Offensive coordinator Dana Bible has also been impressed with Thornton's ability to come in and contribute right away.
"He is learning," said Bible. "He's a true freshman, and in this style of offense you can't be one-dimensional. You catch the ball, you have to block for the quarterback... so there is a lot of things you have to do. He's had to make that transition quickly, and he's really worked at that."
Most importantly, Thornton just gets it. He is humble, values winning, and always credits his teammates... and even rewards them, for his individual success. For example, when he rushed for 100 yards against Wake Forest, offensive lineman R.J. Mattes said that he bought the offensive line doughnuts for their effort.
He is a team-first guy all the way, and NC State knew it when they recruited him out of Liberty County (GA) High School. O'Brien even referenced his leadership abilities on signing day in February.
"We fell in love with Shadrach [Thornton]," O'Brien said at the time. "He is a great kid, and he's going to be a great leader for this football team.
"He's a really good football player. He's the kind of guy that is going to help this football program for a long time."
Bible, when asked about Thornton's leadership qualities, smiled and echoed O'Brien's comments.
"What we love about him is he's all the way in," said Bible. "There is no halfway when it comes to football. He has a passion for this game, and you can see it in his play.
"When he runs, he's having a lot of fun."
Thornton now has three 100-yard games in his freshman season and will be going for his fourth on Saturday in the season finale against Boston College.
Pack fans should be thrilled to know they will get the chance to see Thornton run for a long time.