Image source: Fitbit.

Fitbit's (FIT) newest fitness tracker hit the market earlier this year, and there's already at least one Wall Street pro waxing negative on initial demand. Pacific Crest analyst Brad Erickson is downgrading shares of the leader in wearable fitness, taking his rating from the neutral "sector weight" to the bearish "underweight" on concerns that Fitbit Charge 2 isn't selling so well in its first week or retail availability.

Charge 2 -- the update to Fitbit's top-selling Charge HR bracelet -- isn't selling so well out of the gate, according to Erickson. Pacific Crest spoke with 15 U.S. retailers and the channel checks revealed that inventory was building up. Sell-through should naturally improve as we get closer to the holiday shopping season, but for now some chains have more than two weeks of inventory in stock.

Erickson expects Fitbit to have shipped between 600,000 and 800,000 Charge 2 trackers for the quarter that ends later this week. This doesn't mean that Fitbit is trying to pad this quarter's results by stuffing distributor shelves, but if that is the case, it will be interesting to see if Fitbit updates its guidance when it reports financial results. 

Erickson sees the run rate of Charge 2 clocking in below where the Alta fitness tracker and Blaze smartwatch were earlier this year, and that's not comforting given the strong success that Charge HR had for Fitbit. Another troubling nugget in Erickson's note is that many people that buy Fitbit bracelets stop wearing them after a few months, leading him to believe that they are less likely to buy into the niche product again.

We'll see how it plays out. Erickson wasn't a bull before, and now he's less encouraged. He's lowering his projections for 2017, and his price target of $11 is well below where the stock is now.

An analyst fights back

It also didn't help that Garmin (GRMN 0.73%), the GPS specialist that's been making headway in wearables, introduced a kid-friendly line of fitness bracelets this morning. Garmin rolled out vivofit jr., colorful activity trackers that track steps, sleep, and general activity. The $80 wrist huggers come with a one-year battery life and a parent-controlled mobile app to monitor progress. 

Garmin's gunning for a market that's been tough to crack. Learning toy giant LeapFrog rolled out LeapBand at half the price of vivofit jr. two years ago, and it flopped. Fitbit has fitness bracelets at the $80 price point, but they're not specifically designed for kids.

The stock's trading lower on Thursday -- more on the analyst downgrade than Garmin's new gadget -- but another Wall Street pro did chime in to defend his bullishness on Fitbit. Mizuho's channel checks show that initial sales have been strong and that reviews have been favorable. Mizuho feels that Charge 2 will sell well into early next year, and he views today's pullback as a buying opportunity. 

Investors may be torn about who to believe, but it's fair to say that as Charge 2's popularity goes, so will Fitbit stock. Only one of them can be right about Fitbit's biggest product.