The S&P 500 has been showing signs of weakness to start September, down by more than 1% so far this month. And with many stocks in the index trading at highs, there's definitely room for more of a correction to take place. Given that the index has soared more than 50% since March 2020, it's easy to see why some investors may be considering cashing out any gains they have right now.

That said, there are a couple of stocks that have been struggling this year but could still be underrated buys heading into the final months of 2021. There's reason to be bullish on both ChemoCentryx (NASDAQ:CCXI) and Beyond Meat (NASDAQ:BYND), as these two growth stocks show loads of potential not just in the back half of the year but over the long term as well.

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1. ChemoCentryx

Shares of biopharmaceutical company ChemoCentryx fell off a cliff earlier this year after a key drug, avacopan, which treats autoimmune and inflammatory diseases, failed to receive an overwhelming vote of confidence from an advisory committee for the U.S. Food and Drug Administration. Although the drug completed phase 3 trials for a rare form of vasculitis (inflammation of the blood vessels),  the advisory committee was split on whether the resulting data supported its effectiveness.

The drug could generate close to $2 billion in peak annual revenue for ChemoCentryx according to analyst estimates,  which would be a huge windfall for a company that over the past 12 months reported just $21 million in sales (while incurring losses of $123 million). As with many biotech companies, there's a lot that rides on drug approvals. And while the news wasn't encouraging for avacopan, the FDA still hasn't formally made its own decision. ChemoCentryx made an amendment to its new drug application for avacopan in July and expects that the FDA will make a decision by Oct. 7 -- which is the drug's Prescription Drug User Fee Act (PDUFA) date

A positive outcome there could quickly reverse ChemoCentryx's fortunes. At one point this year, shares of the healthcare stock were trading at more than $70. Today, they're less than $20. And while it's not clear whether it can get back to its previous highs, a green light from the FDA could quickly double or even triple the stock's price -- and set it on a much more positive trajectory heading into next year and beyond.

Although ChemoCentryx is a risky buy, the company has made adjustments to its New Drug Application that should address the FDA's concerns. This stock could be a calculated risk worth taking, given its upside potential.

2. Beyond Meat

Beyond Meat has had a volatile year in 2021, given that the state of restaurant reopenings and the strength of the economy are inevitably linked to this stock's performance. Its plant-based burgers have been a hit in grocery stores and restaurants, but if lockdowns are in effect and consumers aren't eating out, the stock will suffer. (Year to date, Beyond's shares are down 9%, versus gains of 18% for the S&P 500.)

Rising COVID-19 cases have cast a shadow over what's generally been a promising growth stock. In 2020, sales were up by more than 36% from the previous year, even with lackluster food service revenue that declined year over year. During that time, food service sales accounted for roughly one-quarter of total revenue -- in 2019, it was close to a 50-50 split between both of the company's segments, food service and retail.  

If food service can recover, there could plenty of growth ahead for the business. Earlier this year, Beyond reached an agreement with fast-food giant McDonald's to supply the patty for its new plant-based burger. Plus, it announced new meat-free chicken tenders that will be available in U.S. restaurants.

So, while Beyond Meat's stock is down this year, I wouldn't write it off as a bad buy. As vaccination rates rise and (hopefully) COVID-19 numbers start to taper off in the near future, this could be an excellent recovery stock to hold on to. And even if the stock falls further in the weeks ahead, that will only make it an even better buy for the long term.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.