Vikings receiver Adam Thielen once struggled to turn heads in Minnesota.

Now the nation can’t ignore him.

Patrons at the Fort Myers, Fla., bar, where Flint Motschenbacher took a break from the vacation sun to watch his former high school star, were in awe when No. 19 continually made catch after catch at Lambeau Field last weekend. Thielen enjoyed a career day with 12 catches for 202 yards and two touchdowns to put an exclamation point on a breakout campaign in his fourth NFL season.

“It was kind of funny. They were saying, ‘Jeez, that kid — he’s a good player, he’s a good player. They have to keep him,’ ” Motschenbacher recalled. “I didn’t say nothing.”

But he was proud.

Motschenbacher, a 31-year teacher and coach at Detroit Lakes High School where Thielen attended, recalls a plucky, 150-pound receiver who graduated as the program’s all-time leading scorer in basketball.

“You could see the hands,” Motschenbacher said. “He just never had the size.”

Eight years and 50 pounds later, Thielen has put together the best receiving season (960 yards) by an undrafted player in Vikings history. And he’s on the verge of becoming the team’s first 1,000-yard receiver since Brett Favre connected with Sidney Rice in ’09.

Thielen’s story has grown from feel-good to can’t-miss. He’s been the only pleasant surprise for the Vikings’ 30th-ranked offense. But don’t say Thielen, who played Division II football at Minnesota State Mankato, came out of nowhere.

He came from Detroit Lakes, Minn., where No. 19 Vikings jerseys are now common in the school’s halls, Motschenbacher said. Monday morning conversations about Thielen’s big Sundays have grown increasingly common as well.

“There’s a lot of pride in him around here,” Motschenbacher said.

Now the Vikings’ leading receiver, Thielen says fatherhood has been his only lifestyle change during a breakout season. Diapers are certainly one way to keep a guy humble while helping his wife, Caitlin, care for Asher John, their newborn son.

“I’m always going to feel like the underdog,” Thielen said. “I feel like that’s the kind of mind-set I want to have and if I do lose that mind-set, I want somebody to slap me in the face and say, ‘Hey, pick it up and get back to the roots.’ ”

The 26-year-old has had many chips placed on his shoulder, so perhaps it’s not possible for time to erode them.

Thielen wasn’t recruited much as a four-sport high school athlete, needing only one college season to turn a $250 non-scholarship offer at Minnesota State Mankato into a full scholarship. Years later, he still recalls the Mankato coach who wanted him cut.

He wasn’t invited to the NFL scouting combine. He wasn’t drafted. He only received a tryout offer from the Vikings, which he flipped into a contract with no signing bonus or guaranteed money.

Now the soon-to-be restricted free agent on a one-year, $600,000 deal is set to earn his first guaranteed million.

The Vikings sound willing to pay him.

“I think because he’s such a good route runner,” offensive coordinator Pat Shurmur said, “the ball sort of goes his way, and I think that goes back to trust.”

Motivation also keeps finding Thielen.

Leading into the Vikings’ Thursday night game vs. Dallas, a national reporter kept asking Thielen about Cowboys slot receiver Cole Beasley — comparing the two.

Thielen is only 6 inches taller, 26 pounds heavier and plays a different receiver position, but the whole white-and-undrafted link can get in the way of reality.

“I think that’s funny,” Thielen said. “It just shows people don’t watch you play, they just see the stats. They just see ‘undrafted’ and come from a small school.”

As recently as this past summer, some NFL defenders didn’t understand what they were seeing, either. Bengals starting cornerbacks refused to go against Thielen in one-on-one drills during August joint practices, boasting they wanted tougher competition.

“That’s what I feed off. I love that,” Thielen said. “I love when people think I’m not a good receiver. I love when people think it’s going to be easy to guard me.”

If Thielen keeps this up, there won’t be any doubters left.