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

What: Shares of UnitedHealth Group (NYSE:UNH) fell 2% in pre-market trading Thursday after Jefferies downgraded the health insurance giant from buy to hold.

So what: Along with the downgrade, analyst David Windley reiterated his price target of $87, representing just 5% worth of upside to yesterday's close. So while momentum traders might be turned off by UnitedHealth's price strength in recent months, Windley's call could reflect a sense on Wall Street that the company's valuation is becoming a bit stretched.

Now what: According to Jefferies, UnitedHealth's risk/reward trade-off isn't too attractive at this point. "UNH's premium to large cap peers has widened to nearly 2.5x vs historical 1.5-1.8x average. We believe the multiple stays flat at best given below-peer EPS growth (flat and 9% vs peers at 8% and 11% in 2014/15, respectively)," said Windley. "While Optum is performing very well, we believe the best case UNH EPS growth for '15 is in line with peers and likely below. We do not have specific fears about UNH's 2Q, but do not expect significant upside." When you couple UnitedHealth's still-reasonable forward P/E in the low teens with its durable competitive moat, however, long-term shareholders might want to take that short-term bearishness with a grain of salt.