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
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.