Some investing decisions seem like no-brainers. If you know that a company's sales growth is about to evaporate, staying away from the stock makes a lot of sense, right? No one wants to be stuck with a loser.

But some of these decisions aren't as clear-cut as you might think. Pfizer (PFE 0.11%) is at the cusp of entering a sales slump. Everyone knows it. However, I think that Pfizer is a solid pick to buy despite -- and perhaps even because of -- its imminent revenue decline. 

$100 bill cut to form an arrow that's pointing downward

Image source: Getty Images.

A guaranteed slump

The reason why Pfizer is about to experience a sales slump is simple: The company's blockbuster nerve pain drug Lyrica is losing its patent exclusivity. Pfizer managed to delay the inevitable for as long as it could, but the core underlying patent for the drug expires at the end of June.

How significant will the impact be? Pretty big. Lyrica generated nearly $1.2 billion in revenue in the first quarter of 2019 and more than $3.8 billion last year. Those sales won't evaporate instantly, but Pfizer warned in April that it expects "to enter a period of significantly reduced revenue." 

You might think that perhaps Pfizer's other products could offset the sales decline for Lyrica. Nope. Pfizer's total revenue in Q1 increased by only $212 million year over year despite strong growth for anticoagulant Eliquis and breast cancer drug Ibrance. What about the big pharma's acquisition of Array BioPharma (ARRY)? It won't provide any substantial financial benefit anytime soon.  

It's pretty much guaranteed that the company is about to flip from sales growth to sales declines. Wall Street analysts estimate that Pfizer's revenue will fall by around $290 million in the third quarter compared to projections for Q2.

Beyond 2020

With this bad news on the way, it's not surprising that Pfizer's share price has slipped a little year to date while the broader market indexes have soared. Investors know that the company will soon feel the sting from Lyrica's loss of patent exclusivity. Pfizer's stock price reflects it.

When investors look to the future, they sometimes have a limited vision. Yes, Pfizer will no doubt hit a rough patch in the second half of this year and into 2020. But beyond 2020, the outlook for the drugmaker improves considerably.

Pfizer CEO Albert Bourla stated in the company's Q1 conference call that the Upjohn segment, which is home to Pfizer's drugs that have lost patent exclusivity or will do so soon, should generate stable low-single-digit top-line growth after 2020 when the impact of Lyrica's patent expiration has been absorbed.

This projection seems achievable, in my opinion. Upjohn's potential in emerging markets, especially in China, should help the segment become a modest revenue growth driver for Pfizer and an even greater contributor to the company's profitability.

Pfizer's biopharmaceuticals segment should deliver much stronger growth in the next decade. I think that Ibrance will enjoy a second wave of momentum in the adjuvant setting for breast cancer. I don't see any significant bumps in the road for Eliquis. And while Merck hopes to launch a rival to pneumococcal vaccine Prevnar 13, Pfizer's patent portfolio is strong and should allow it to hold onto market share until it launches its own successor to the blockbuster vaccine. 

In May, the FDA approved Vyndaquel and Vyndamax for treating the rare genetic disease transthyretin amyloid cardiomyopathy. The two drugs should combine for peak annual sales of $2 billion. Pfizer also picked up another FDA approval recently for a combination of Bavencio and Inlyta in treating advanced renal cell cancer. 

Pfizer's pipeline also includes more than a dozen other programs with blockbuster potential. The acquisition of Array should add two other big winners, with the biotech's Braftovi and Mektovi picking up momentum in treating melanoma and showing promise in combination with Erbitux in treating colorectal cancer.

Why not wait?

There's a lot of good news in store for Pfizer after the bad news resulting from Lyrica losing patent exclusivity. So why not just wait until sometime next year to think about buying the stock?

First, remember that while investors might not always look far enough into the future, they're always looking to the future. You can bet that Pfizer's improving prospects for beyond 2020 will be reflected in its stock price at some point in 2020. But there's no way to know for sure how far in advance investors will realize Pfizer's Lyrica-caused sales slump doesn't tarnish its overall long-term prospects.

Second, Pfizer pays you to wait. The company's dividend yield currently stands at 3.3%.

Yes, a period of malaise is about to hit Pfizer's top line. But some are focusing too heavily on Pfizer's temporary near-term troubles instead of the company's solid long-term growth prospects. And that gives investors willing to question the conventional wisdom a great opportunity to buy this big pharma stock.