"The stock's recent run-up fails to reflect that a higher price paid for Fox's Entertainment assets would reduce the value of Disney to its shareholders," wrote analyst Brian Wieser. "Alternately, the absence of completion of the transaction would also be negative for Disney as it would mean the company would be unable to realize the synergies it expects to produce from the transaction."
Disney's shares are up 9 percent this month amid the battle for Fox and hopes of overall industry consolidation in the wake of a judge's decision on Tuesday to allow the AT&T-Time Warner merger proceed. On Wednesday, Comcast bid $65 billion for the Fox units currently set to be acquired by Disney. Comcast's cash bid represents a 19 percent premium to Disney's offer in stock.
"For the moment we are leaving our valuation of Disney and leaving the assumptions incorporated into our model unchanged. But because we don't see much, if any, upside to our prior $93 price target, we are downgrading the stock from Hold to Sell at this time," the analyst wrote.
Shares of Disney fell 1.4 percent in Monday's premarket to $107.30. They closed Friday at $108.85.
Disclosure: Comcast is the parent company of NBCUniversal and CNBC.