United were thumped 3-0 at Fulham on Saturday with a make-shift defence.
Ferguson, however, refused to blame his players for the rout.
"There was a fragility at the back and we couldn't do anything about that," said Ferguson. "You have to have sympathy for the players in that respect.
"It's difficult to play midfielders at centre back and Ritchie de Laet is only 21 years of age. He's not got the experience yet needed for these type of matches.
"We couldn't get behind Fulham at all and their defence is very well organised and it was a difficult day for us.
"The players are maybe feeling a bit sorry for themselves at the moment because of the situation they're in.
"But the quality of the football wasn't as good as it normally is.
"The fragile nature of the players at the back, with midfielders playing in defence, has left them feeling sorry for themselves a bit."