Morgan Stanley is concerned about 5 red flags with Under Armour's business

Under Armour has been nearly unstoppable - until now.

With celebrity endorsements from megastars Steph Curry and Jordan Spieth, company has been touted as a viable competitor to Nike.

But now, Morgan Stanley's analysts are becoming concerned about the state of the business and believe that the company will report apparel sales that are flat or negative.

Here are five red flags Morgan Stanley sees in the business.

1. Sales growth has halted.

Under Armour has posted impressive sales growth for the past few years. But now, sales seem to be going flat-to-negative. It's possible that Under Armour grew so much that it has saturated the US market.

2. Shoes are selling at a discount.

Under Armour's running footwear is selling for 23% less than two years ago. Prices across the industry have declined only 4%.

If Under Armour wants to catch up with Nike, it will need to communicate why discerning runners should buy its shoes instead of the competition.

3. Outlets are overtaking the business

Under Armour's primary growth opportunities are in off-price outlets and in family footwear, according to Morgan Stanley. This is inconsistent with the high-end activewear image Under Armour is trying to cultivate.

4. Women are turning away from the brand

Morgan Stanley analysts believe that women are buying from competitors like Lululemon, Nike, and Athleta instead of Under Armour. In recent years, many athleticwear companies have started catering more to women's tastes, leading to increased competition in the space.

5. Inventory is growing fast

Inventory expanded by 46% in the fourth quarter, compared with a forecast of 29% sales growth in the first quarter of 2016. This means that Under Armour will have to potentially discount to clear out the excess merchandise, hurting profits.