Argentina's Acasuso to replace injured Del Potro in Davis Cup finals
Del Potro pulled a muscle in his right leg during his loss to Feliciano Lopez on Friday.
The 20-year-old, who rocketed into the top 10 this year, had already been suffering from a toe injury.
"I could have gone out and tried but I didn't want to risk having to pull out after a few games," Del Potro said in a television interview.
His replacement Acasuso, 26, is ranked 48th and has won five out of seven deciding singles matches in the Davis Cup.
Ferrer was outclassed by David Nalbandian in Friday's opening singles while Verdasco, 16th in the ATP rankings, was inspirational as he helped Lopez to win the doubles on Saturday.