Gradi, who enjoyed a three-year spell as manager of the Dons from 1978 just 12 months after they were elevated to the Football League, insists that meeting the south Londoners represents the toughest test he has tackled during his 13 years in charge of the Second Division club.
"Wimbledon are the highest-placed League team we've ever faced. It's like mission impossible for us, but you can never rule us out of doing it and a trip to play Manchester United in the fourth round is the biggest and best incentive we could have."
Gradi is set to make at least three changes, with Jamie Moralee injured and Peter Billing and Billy Barr under under pressure for their places. Long-term casualties Ben Thatcher and Paul Heald are missing for Wimbledon and Brian McAllister is standing by for Dean Blackwell, who has a thigh strain.
Derby County, having already been put out of the Coca-Cola Cup by Luton Town, will be desperate to avoid another upset at Gillingham, who removed Coventry City from that competition earlier this season.
However, the Derby manager, Jim Smith, has to find a replacement for Igor Stimac after Dean Yates damaged a knee in Saturday's 1-1 draw at Wimbledon. With the Croat centre-back suspended and out for five weeks after a hernia operation, Yates was having a scan on the knee yesterday, fearing a lengthy spell out of the game.
Smith has recalled Matt Carbon to play alongside Paul McGrath and Gary Rowett at the back in tonight's third-round tie.
Gillingham, though, are without their top three strikers, including the pounds 250,000 signing, Ade Akinbiyi, who joined them too late from Norwich last week to be eligible for this round. Iffy Onuora starts a four-match ban for his second sending-off this season, at Burnley last month, and Leo Fortune-West is still recovering from a broken leg.
Yet the manager, Tony Pulis, who is likely to pair Dennis Bailey and Steve Butler in attack, insists: "We are looking for another big cup result to get us going again in the league."