Carroll was loaned out to the Hammers earlier this season, having joined Liverpool from Newcastle in a big-money deal in January 2011.
Ferguson respects the 23-year-old for his physical qualities but admits he found it surprising that the Reds paid so much for a relatively unproven player.
"He'd played not a lot of games for Newcastle and had to deal with a £35m transfer to a club like Liverpool," Ferguson said in the Sunday Mirror.
"He was young and it was quite a big jump for him for that kind of money. We've gambled on young players in the past, a couple of million, something like that.
"But when you get to that level of £35m, you need to do a bit more homework and see how he is in the second season, maybe.
"Now it's up to the boy to improve himself. He's a great header of the ball and very aggressive. He'll be a handful on Wednesday."
Ferguson has also suggested that Brendan Rodgers showed his inexperience by letting Carroll link up with West Ham on deadline day before he had signed a replacement.
"They were after Clint Dempsey, that didn't work out, and they had done the deal for Carroll," said the Red Devils boss. "Maybe it was something they did not expect.
"With the manager being young, maybe he expected the deal to go through and then it was not there and he had lost a player, too."