Jeffers becomes No2 to new Macclesfield manager John Rooney, the younger brother of Manchester United legend Wayne Rooney.
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Rooney has taken charge after former manager and Macclesfield shareholder Robbie Savage left to take the job with League Two Forest Green Rovers.
Jeffers said: "It's a massive opportunity. The gaffer is someone I've known for a long time.
"I've coached him, so I know what he's about. So, I'm really looking forward to it."