England skipper Ferdinand damaged knee ligaments in training when he tackled Heskey and the Aston Villa striker fell on his left leg.
The incident happened on Friday and was described as a freak accident by Ferdinand.
However, Heskey has found it hard to cope.
The striker said: "I spoke to Rio afterwards and he told me there was nothing I could do but it was still difficult for me.
"What happened was that I tried to turn the ball round the corner and fell backwards onto him but his leg was in a position where he couldn't actually move it.
"There was nothing you could do.
"Knowing he is out of the World Cup was very difficult.
"He is our captain and has been a great player through the years for England and at club level.
"It was just one of those things."
Heskey is being tipped to start Saturday night's World Cup opener against the United States despite a record of just seven goals in 58 appearances for England.
Boss Fabio Capello believes the 32-year-old causes major problems for opposition defences, wearing them down with his physical style.
And Wayne Rooney and Steven Gerrard always seem to be more effective when the burly hitman is playing.
Heskey conceded: "Forwards are judged on their goalscoring.
"People do look at that. But I like to think I bring a lot more to the game and I do get pleasure from assisting.
"I also get pleasure from being part of a winning side.
"I do sometimes think I could have done better in certain situations and got a goal myself but that is part and parcel of the game."
FUCK HESKEY!!! A MANCHESTER UNITED PLAYER IS TO LEAD ENGLAND TO WORLD GLORY,NOW ITS THAT FUCKING LIVERPOOL GAY THE NEW CAPTAIN.FUCK HESKEY!!!