The twice former champions have already qualified for the last 16 but narrow wins against debutants Bosnia and unfancied Iran have been less than convincing for a team who were among the favourites going into the tournament.
Several chances were squandered against Iran before Lionel Messi, who was celebrating his 27th birthday yesterday, grabbed the only goal with a stoppage-time winner.
"I'm worried in the sense we have to improve," Sabella said. "We need to be more mobile, to find spaces and we will see if the match will be different. We are going to meet a team that can play with high potential, switch to attack with ease."
Sabella said he had not yet picked his team because some players - he did not say who - still had injury worries but he added that he would likely play a 4-3-3 formation.
Although Nigeria have also struggled to shine, Sabella said he expected a tough game against the African champions. "I see a fast team attacking swiftly," he said. "They are strong when they get the ball into attack. It is a team that is very fast and that makes you worry. We have seen some surprises but that doesn't mean we will have one tomorrow."
Nigeria are the only side at the tournament who have yet to concede a goal, and asked if Argentina would rely too much on Messi to provide the first, Sabella said: "Every time you have a player like Messi there is a dependence. So we try to reduce this dependence but you are always dependent on a player like that.
"This pressure is normal for a great player. We try to take some weight off his shoulders. We are a team, we have to help each other."