Moyes has spent almost 11 years with the Toffeemen and is widely regarded as one of the best managers in the Barclays Premier League.
But his contract is due to expire in the summer and the Scot has been linked with a host of top jobs, including Manchester United when Ferguson eventually decides to call it a day.
Understandably, Everton chairman Bill Kenwright is keen to keep Moyes on Merseyside, and the chances are, in the short-term at least, that is where the Scot will stay.
However, after spending so many years punching above their weight, Ferguson feels Everton may be in trouble should Moyes walk away.
"It's obviously a concern for Everton," said Ferguson.
"It would be a big blow because he's a good quality manager."
