The former United manager, now a club director and ambassador, says that Foulkes was a topic of conversation between himself and Sir Bobby Charlton only Sunday.
"I was very sad to hear the news," Sir Alex told ManUtd.com. "Bobby and I were talking about Bill yesterday on the way down to Cardiff. He was a really nice man and a great servant to the club, too.
"When I first came in as manager, Bill was managing in Norway and he used to bring players over to training at the Cliff. He came to training quite a lot and I got to know him well through that. He was such a nice, quiet man to know.
"The story of his life was absolutely incredible," he said, "and he's assured of his place in our history by his appearances and by the way he performed, particularly in the aftermath of the Munich air disaster.
"Having gone through that, how he and Harry Gregg managed to perform a couple of weeks later, leading those young lads out against Sheffield Wednesday - and winning the game - was absolutely incredible. He was an exceptional man."
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