The 20-year old winger will join United in the summer after agreeing a pre-contract with the English champions in January but Wednesday's retirement announcement from Ferguson means the Eagles young gun will not the chance to play under the Scottish great.
"It's the end of an era," Holloway said.
"A fantastic, great man who will bow out at the top. The best manager in my lifetime has suddenly stopped.
"I feel sad for Wilf as well as I'm sure he wanted one or two seasons with him.
"I had a chat with Wilf and I'm sure when it all settles down they will be in touch. He goes down as Sir Alex's last ever signing - it's not a bad one."