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While Under Armour Inc. (UAA) shares were surging Wednesday afternoon, there's one big long-term risk that looms as especially pertinent for investors to consider. 

If Under Armour doesn't hit a certain EBIT (earnings-before-interest-and-tax) margin, the stock may be perceived as a dead end for a long time, Morgan Stanley analyst Lauren Cassel wrote in a note late Tuesday evening. 

As for Wednesday, the stock rallied 6.47% to $23.21 each. The stock was up a bit in the morning, before really popping around noon, when Federal Reserve Chairman Jerome Powell spoke at The Economic Club in New York, and came off as slightly more dovish than he has all year.  

Under Armour has crushed the broader U.S. stock market year-to-date, at a 60.71% increase, with a trailing 12 months price-to-earnings ratio of 186, an enormous valuation. Now, investors need to watch those operating margins and revenue growth on a longer-term basis. 

"Low- to mid-single-digit revenue CAGR [compound annual growth rate] guidance and a +MSD EBIT margin target underwhelm and fuels the 'Under Armour is a permanently impaired brand' bear thesis," Cassel wrote. "If UAA can't achieve more than a ~6% EBIT margin over the next five years, we see downside risk." That's built into her bear case, to which she assigns a 20% probability. 

Sure, Under Armour has performed well this year, but the last three years as a whole have not been pretty for the sportswear seller. The stock is down 52% in that span, and down about 55% from its all-time high reached in September of 2015. In the past 5 years, the stock is up a measly 11%.