During his two seasons as Ronald Blair’s position coach, Appalachian State’s Mark Ivey often gave the rest of his defensive linemen an order during video-review sessions: Don’t do what Ronald just did.

That meant the rest of the Mountaineers shouldn’t attempt to bull-rush offensive tackles who outweighed them by 70 pounds. And they shouldn’t vacate their position after the snap to assist a teammate in need of assistance.

“Sitting in our room last year I would say, ‘Fellas, don’t get yourself upset — I don’t want anyone else trying that because it’s not going to work,’” Ivey said. “Ronald can and I’m going to let him continue because he’s physically capable.”

Blair was, indeed, capable of great feats against the likes of Louisiana-Lafayette. But his lack of height and elite college competition raised concerns about how the Sun Belt Conference Defensive Player of the Year would fare in the NFL.

Four months later, the 49ers’ rookie defensive tackle has generated training-camp buzz rarely attached to fifth-round picks. In fact, after doing what his teammates weren’t allowed to do in college, it appears he could do what so many have failed to do in the NFL: Thrive as a 6-foot-2 defensive lineman.

Consider that defensive coordinator Jim O’Neil has spoken of the 142nd overall pick as a rookie whom opponents will have to account for. O’Neil has said Blair is a rare triple-threat, capable of also lining up at nose tackle and outside linebacker after playing four defensive-line positions in college.

“He’s going to be a guy that we move around,” O’Neil said. “Offenses are going to have to find him and deal with him.”

A bit much? Perhaps. But it’s not just O’Neil. Blair’s veteran teammates are also part of the chorus. When it was noted Blair (270 pounds) didn’t look like behemoth first-round defensive tackles DeForest Buckner (6-7, 300) and Arik Armstead (6-7, 292), safety Eric Reid didn’t hesitate.

“But he plays like it, doesn’t he?” Reid said. “I don’t watch a ton of film on him, but what I do see from him is he’s in the backfield. You look up and he’s touching the running back as soon as he gets the ball.”

When it was noted how effective Blair has been this summer, All-Pro linebacker NaVorro Bowman sounded genuinely excited.

“I was going to use the word ‘effective,’” Bowman said. “That’s exactly what he is. He’s effective. He’s disruptive. He’s a guy that knows how to use his size.”

Oh, yes, his size.

Only 11 of the 64 defensive linemen at this year’s combine were shorter than Blair, but he’s been able to compensate for his lack of height with gas-hose arms (34 inches). And he’s labored to ensure his lack of height was offset by strength: Blair’s 32 reps of 225 pounds on the bench press tied for second among defensive linemen at the combine and ranked fourth overall.

But his best asset could be his instincts. Niners defensive line coach Jerry Azzinarro says Blair has a natural understanding of concepts, spacing and leverage, saying, “He’s just one of those guys that has the ‘it’ factor and they can just feel the game.”

Just ask Georgia Southern, which features a run-heavy, triple-option offense. In video study last year, Blair recognized the Eagles quarterback always pitched the ball outside if the middle appeared clogged. So he pretended to be defending against an up-the-gut dive before leaping outside to greet a running back.

“Ronald would do things that weren’t exactly the way I’d coached it to him,” Ivey said. “But when I asked him why he did it, his rationale was so good that some of the stuff we implemented into our defense.”

Blair has inspired plenty of praise this summer, but his production has been lacking in two preseason games (1 tackle, 1 quarterback hit in 74 snaps). O’Neil has said the stat sheet has been misleading and Blair has routinely generated pass-rush pressure that’s allowed others to shine.

“Everywhere we put him,” O’Neil said, “he makes plays.”

For his part, Blair expects more and insists he’s retained his feisty underdog attitude despite receiving plenty of praise. After he was lightly recruited out of high school and lightly regarded in the draft, he’s doing the heavy lifting required to silence more skeptics.

“Getting (big-headed) is the last thing on my mind,” Blair said. “I have so many more improvements to make and so much work to do. It’s a tremendous honor to have guys praise me, but I know I have such a long ways to go. I think I’ve just shown a glimpse of some of my talents.”

Eric Branch is a San Francisco Chronicle staff writer. Email: ebranch@sfchronicle.com Twitter: @Eric_Branch