However, he said the group may return once it sees signs of trading starting to stabilize. "We downgraded because one of the issues was that we didn't think a deal would happen as quickly as people thought," he said.
Vodafone is the world's second-largest mobile operator, with assets in Europe, India and Africa. Britain's Sky News reported on Saturday that AT&T had started courting European regulators, with Stephenson meeting the European Union telecoms commissioner Neelie Kroes.
AT&T is the second-largest operator in the U.S. after Verizon Wireless, the joint venture that Vodafone is in the process of selling out of for $130 billion.
(Read more: Why $130 billion Verizon-Vodafone deal makes sense)
But it is not adding new customers in its home market as fast as Verizon, and it is also ceding market share to much smaller rival T-Mobile US.
Analysts and bankers have speculated that the sale by Vodafone of the U.S. Verizon Wireless unit to majority partner Verizon Communications, which is due to complete in February, would leave the British firm more open to a takeover.