Eustace left to work with Steve McClaren at QPR on Friday morning, writes Tom Feaheny.
Now the 40-year-old MacFarlane, the ex-MK Dons and Fleetwood coach, has been picked to take over.
Chairman Colin Gordon knew a quick appointment was needed and is thrilled to get his man.
“He’s been here before and I know the way he works,” he told the club’s website.
“He fits the profile of the philosophy of what we’re looking for. He’s hard working, fun and studious, with very good experience and great confidence.
“He’s done his research and has watched a numerous amount [of our games] games and already has his ideas on how he would like to work and how he would like to play.”