Parreira fielded Adriano and Ricardo Oliveira together in a training session between the first team and the reserves, then said he was still considering throwing 21-year-old Santos striker Robinho into the fray.
Absent from the session because he is away on club duty, Robinho is regarded as Brazil's most gifted young forward, although he has relatively little experience against top-level opposition.
"This is the doubt Robinho and Adriano could start together in attack, but Ricardo (Oliveira) is also an excellent option," Parreira said.
Ronaldo had asked to be excused the Confederation's Cup in the second half of this month but Parreira decided to leave the Real Madrid striker out of next week's World Cup matches as well.
His absence has opened the way for Adriano, who has been mainly used as a substitute for his country despite an excellent season with Inter Milan, to get a place in the starting line-up.
Second in the 10-nation South American World Cup qualifying group with 24 points from 13 games, Brazil are at home to Paraguay in Porto Alegre on Sunday and away to arch-rivals and group leaders Argentina three days later.
Parreira said he was also undecided about which two central defenders would be selected from Juan, Lucio and Roque Junior.
The coach said he expected Paraguay, who are attempting to qualify for their third successive World Cup, to defend in depth and play a long-ball game.
"It's a team who are not ashamed to defend with nine or ten players and are not ashamed to take the direct route all the time," he said.
