"We are having the good fortune that other teams are not," Demichelis told a news conference at Argentina's base in the University of Pretoria sports complex.
"We've had no injuries and that gives the coaching staff the confidence to demand the maximum from us," he added after Diego Maradona's squad practised in front of a crowd of about 3,000 Argentine and local fans on Sunday.
"I'm confident that in the remaining days we'll be able to get there in very good shape," he said of the match at Ellis Park in Johannesburg.
The build-up to the tournament that kicks off on Friday has been hit by a spate of injuries to leading players including Nigeria midfielder John Obi Mikel.
"Some teams have played too many friendlies. Luckily Diego opted to come here and train in tranquility," Demichelis said.
In a reference to the form of the Argentina players, he said: "Diego has a big headache picking his side while Nigeria have doubts.
"I think that they may have some issues to resolve in some positions and won't get there (to the match) as good as us."
The crowd allowed in to watch, including groups from local schools, learnt favourite chants from Argentina fans while waiting for the practice to start.
They then gave the squad and particularly Lionel Messi and Maradona a rapturous welcome when they came out onto the pitch.
Demichelis said such attention on Maradona from fans and media, also out in great numbers, helped release the pressure on the squad.
"Obviously, Diego decompresses the situation...(His fame) is also a motivation and we enjoy that. We are very happy to have him as our trainer."
The squad were in full at the practice, striker Diego Milito having recovered from a twisted ankle on Thursday and defender Ariel Garce from a knee problem.
Maradona organised a 40-minute game on a shortened pitch with his first choice attackers facing a majority of the defenders likely to play on Saturday.
There was not too much worth reading into the line-ups though with Messi, Carlos Tevez, Gonzalo Higuain and Milito all on the same side and Maradona set to pick three of them at most.
