It has been a brutal year so far for Under Armour (UA) (UAA) , with shares falling roughly 50% in 2017. Making matters worse, Nike (NKE) stock has been on fire since its October lows, climbing from almost $50 to over $64. Shares are now up 27% on the year.

But Under Armour's time could be coming.

According to Jim Duffy, an analyst from Stifel, shares of Under Armour could rebound in 2018. He upgraded the stock to buy from hold and assigned a $17 price target. Depending on whether investors are trading UAA or UA stock, that implies about 23% to 36% upside from the stocks' closing prices on Thursday.

Shares are up roughly 4% in response.

Duffy contends that Under Armour's business should be "in a much better position entering 2019" vs. where it's at now entering 2018. Like many other stocks, he believes investors will begin to price these improvements into the stock before they begin to occur, given that the markets tend to be forward looking.

Duffy also argues that a return to "full-price apparel and footwear sales at retail" will help Under Armour, as will a boost in its margins.

But he didn't stop there. Duffy is making all sorts of waves Friday, as shares of Fitbit (FIT) crumble 9% at the open. The analyst downgraded Fitbit to sell from hold, although he maintained his $6 price target. Prior to Friday's trading session, Duffy's price target implied about 12% downside in Fitbit stock. His current target suggests about 5.5% downside.

So why the negativity? He suggests that in order for Fitbit to obtain flat operating margin, the company needs to generate mid-teen sales growth. In his view, that's a "high hurdle," meaning negative operating margins are a real possibility.

Where does Nike stand in all of this? Shares hit a new 52-week high of $65.07 in Friday's session.

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This article is commentary by an independent contributor. At the time of publication, the author had no positions in the stocks mentioned.

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