While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a look at particularly stock-shaking analyst upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of MasterCard (NYSE:MA) slipped about 1% this morning after FBR Capital downgraded the credit card giant from outperform to market perform.

So what: Along with the downgrade, analyst Scott Valentin planted a price target of $82 on the stock, representing about 8% worth of upside to Friday's close. While contrarians might be attracted to MasterCard's earnings-related plunge last week, Valentin thinks the appreciation potential remains limited given the several headwinds working against the company.

Now what: FBR lowered its 2014 earnings-per-share estimate for MasterCard from $3.13 to $3.03, and its 2015 view from $3.76 to $3.65. "The downgrade reflects our concern regarding MasterCard's exposure to potentially slowing emerging market economies, along with increased currency volatility, having a negative impact on MasterCard's results and investor sentiment," Valentin noted. "APMEA and Latin America comprise 37% of MasterCard's [gross dollar volume], and while posting strong current GDV growth rates, we believe they have the potential to slow."

With MasterCard shares off 10% over just the past month, however, those concerns might be providing patient Fools with a high-quality value opportunity.