Plank also praised President Donald Trump's pro-business philosophy and called him a "real asset."
The Baltimore apparel and fitness company's stock price fell after Under Armour announced sales and earnings for the fourth quarter that missed Wall Street expectations and a forecast for 2017 that showed slowing growth.
"You look in my eyes and tell me: Do you believe this is the team that's done this for 21 years, 11 years public and knows how to win?," Plank said on CNBC's "Halftime Report."
"You know, I think a lot of people bet against Tom Brady the other night, too," he said.
Brady -- an Under Armour endorser -- led the biggest comeback in Super Bowl history Sunday against the Atlanta Falcons.
Plank said in the interview that Trump is a "highly passionate" and a pro-business man who is a "real asset" for U.S. companies.
Plank is among the business leaders participating in Trump's ongoing manufacturing jobs initiative. He met with the president and the executives at the White House last month.