Ferguson, who made eleven changes from the team that defeated Chelsea 2-0 in the Premiership on the weekend, called on his youngsters for the third round tie against Iain Dowie's outfit on Wednesday.
However it was a gamble that backfired spectacularly for United as Coventry became the first lower league club to win at Old Trafford since York City in1995, with Ferguson admitting the 2006 winners had no excuses for the shock result.
"I'm flabbergasted by our performance. It is a big shock for us all," Ferguson said.
"These are young players that we have shown great hope for and trumpeted them in a loud way.
"I'm flabbergasted by our performance. It is a big shock for us all."
Sir Alex Ferguson,
Man United manager
"You hope that the young players will look at it and want to improve themselves. Their temperament, against a team who were ready for a cup tie, was certainly an issue in the first-half, so maybe they are not used to that kind of cup-tie football," the Scotsman added.
"One great save by their keeper led to a goal at the other end that won them the game, but that's not the way we should look at it because the other 89 minutes were disappointing.
"But we will keep giving youngsters their chances. We have been doing it for years now and these players have to have to have games."
Old Trafford clean-sheet
Coventry, who copped a 4-1 hammering at the hands of Ipswich in the Championship last Saturday, were always the more impressive team against a United side without a host of star names.
Unheralded players such as Gerard Pique, Lee Martin, Danny Simpson and Chinese forward Dong Fangzhuo all failed to produce the goods, and Coventry boss Dowie admitted Ferguson's team selection helped his side.
"Alex put out a team that was good enough to win, but obviously not seeing players like Rooney, Ferdinand, Scholes and Vidic on the team-sheet is a benefit," said Dowie.
"To talk about players that weren't here, though, would be a travesty for Coventry City. We restricted United to very few out-and-out chances. There was one great save by our keeper, Andy Marshall, but that speaks volumes for our defence and our desire.
"To get a cleansheet at Old Trafford against United is a big thing for us and the players have really made me proud. They had players like Carrick, O'Shea, Nani and Anderson on the field from the 55th minute, so it is a very good performance for us."
Other League Cup action
Dean Ashton has now scored three goals in
three matches for West Ham [GALLO/GETTY]
In other matches on Wednesday, Aston Villa also suffered a shock home loss to Leicester City after a Matt Fryatt goal in the 76th minute, while a Stelios Giannakopoulos strike in the 112th minute sealed a 2-1 extra-time win for Bolton over Fulham at Craven Cottage.
Avram Grant, new Chelsea manager, got his first win on the board, albeit a 4-0 away result to Hull with Salomon Kalou scoring a double, while Blackburn and Everton recorded 3-0 victories over Birmingham and Sheffield Wednesday respectively.
In London, West Ham defeated a plucky Plymouth outfit 1-0 thanks to a late Dean Ashton volley, while Tottenham accounted for Middlesbrough 2-0 with second-half goals to Gareth Bale and Tom Huddlestone.
The draw for the round of 16 is on Saturday.