Di Maria limped off the field in the first half of Argentina's quarterfinal win over Belgium after straining a muscle in the back of his right thigh.
Team doctor Daniel Martinez yesterday said tests showed it was a "first-degree" strain -- the mildest kind but added that Di Maria wouldn't be fit to play in Wednesday's semifinal.
Martinez said Aguero, who sustained a similar injury 10 days ago, had recovered in an "excellent" way.
Di Maria scored the extra-time winner against Switzerland in the second round and set up Gonzalo Higuain's winning goal in the first half against Belgium on Saturday.
More From This Section
"He's a key player in this project," Higuain said of Di Maria.
The last time Argentina reached the semifinal stage in the World Cup was 24 years ago in Italy, when it made it to the final but lost 1-0 to West Germany.
Sabella made two key changes to the lineup against Belgium, fielding Manchester City defender Martin Demichelis for the first time in Brazil, and having Lucas Biglia replace Fernando Gago in the midfield.
"You always hope to play," Demichelis said. "I was showing a lot of heart in practice and tried to get to the place where I would get called up."
The moves bolstered Argentina's defense, with Biglia and Javier Mascherano dropping back to help protect the early lead.