Good Under Armour results not good enough: Analyst

Sport's world war: Under Armour vs. Nike
Under Armour's earnings underwhelming
Under Armour reports quarterly results

Under Armour on Tuesday reported better-than-expected earnings and revenue, but analysts wondered whether the first-quarter results were good enough.

"By most measures, this was a very good quarter," said Oppenheimer analyst Brian Nagel. "But I think the question is, is it good enough for the lofty expectations in the marketplace?"

The athletic apparel maker earned 5 cents per share, a penny above estimates.

But results were 13.4 percent below the year-ago period—hurt by higher costs related to the February acquisitions of two fitness tracking services, Endomondo and MyFitnessPal, for about $560 million.

Under Armour is attempting to carve a stronger presence in digital health monitoring to complement apparel sales. MapMyFitness became part of the digital portfolio in December 2013.

Revenue of $804.9 million in the first quarter also beat forecasts.

While Under Armour raised its full-year guidance on net revenue to $3.78 billion and operating income in the range of $400 million to $408 million, those forecasts were not as high as hoped.

As a result, the stock was under pressure in premarket trading. (Get the latest quote here.)

"Given the run in the stock and ... the expectations within Under Armour, there could be some cracks here," Nagel said in an interview on CNBC's "Squawk Box."

Shares of Under Armour have soared 28 percent higher this year—outperforming competitors including Nike and Lululemon.

Read MoreWhy Jordan Spieth's Masters win = Nike nightmare

Even though few companies in the consumer space can post this kind of top line growth consistently, Nagel said he still has only a neutral rating on the UA stock. "Valuation keeps me on the sideline. I think the stock right now is just too rich."

CNBC's Terri Cullen and Jacob Pramuk contributed to this report