The Australian's promotion was announced to staff on Wednesday morning along with news that Simon Dicketts, who has held the job since the agency's foundation seven years ago, will become the executive creative director while taking on a broader management role.
However, the agency insisted that the decision to give Eastwood day-to-day charge of its 12 teams after less than a year in the London office would have been made whether or not Grey had been trying to entice him away.
"It's got nothing to do with Grey," Dicketts said. "This decision has been a long time in the making."
Tiger Savage, the agency's head of art and one of the department's group heads, had long been considered the favourite to succeed Dicketts. But he said: "I've had no resignations, nor do I expect any."
Eastwood's move into the creative hotseat will result in the immediate dismantling of the group system. From now on, all creatives will report directly to him.
Dicketts will continue writing ads but will also become more involved in new business and international matters as well as creative training.
Eastwood, 35, a writer, has been at M&C Saatchi for seven years, working as the creative director of its offices in Melbourne and New York, where he was closely involved in the agency's showpiece British Airways account.
Since his arrival in London he has worked on Natwest and helped to evolve the Daily Mirror's "think again" campaign.
Eastwood said: "Everybody knows I had conversations with Grey but M&C is like a family for me and I couldn't imagine leaving."
Nor was he expecting problems with Savage. "It could have been difficult but Tiger and I have known each other a long time and I rate her as the best art director in London," he added. "I want her to concentrate totally on improving our visual output."