By Jamie McGeever LONDON (Reuters) - If implemented, the last-gasp deal to keep Greece in the euro zone should allow the region's economy and markets to regain the momentum they showed earlier this year by removing an existential threat to the 19-nation-bloc, economist Nouriel Roubini said on Monday. If there had been a 'Grexit' - Greece exiting the euro zone - a "fundamental repricing" of euro zone assets would have been appropriate, said Roubini, the man known as "Dr Doom" who was among the few to predict the U.S. housing crash that triggered the global financial crisis. "It's a constructive deal, positive for the euro zone, and means that for now, those tail risks that would have led to a more fundamental repricing of euro zone assets should not occur," Roubini told Reuters in an interview in London.