They've spent more than £200million assembling the best talent available, on paper at least they should ease past an Iron side - which hasn't cost a penny in transfer fees.
But history actually favours the relative minnows, who've won two of the four past meetings.
And Clarke was part of the Chelsea squad that United knocked out of the League Cup over two legs in 1988 - witha 4-1 win at Glanford Park and a 2-2 draw at Stamford Bridge - so is keeping the Chelsea players' feet on the ground in his role as assistant manager.
He said: "We won't treat them lightly. I've tried to tell the players about that game. I was embarrassed we did not win over the two legs.
"It was a difficult game at Scunthorpe and I think it was 1-1 for a long period. Then the roof fell in and they scored three goals in the last five or six minutes.
'Obviously it was an embarrassing night for us at the time but it was a long time ago and things have changed dramatically at this club since then.
''We felt we could pull it back at home as we had just been relegated to the old Second Division and were going well in the league.
''But it was a big upset for us and a timely reminder for our players that cup upsets can happen."