France warmed up for next month's finals with a less than impressive 3-2 win over Iceland on Sunday after finding themselves 2-0 down to the minnows.
Manchester City ace Nasri admits they need to plug the gaps ahead of their opening Group D clash with England on June 11 or face being punished by Roy Hodgson's side.
"We leave ourselves exposed a lot with the system we have. We struggled to have enough cover in midfield," admitted Nasri.
"Our opponents found gaps to get through. We need to find the right balance if we line up the same way.
"That's because once the Euros get under way it will be difficult for us to come back if we start a game the way we did against Iceland."
Despite their struggles against Iceland, Nasri believes the comeback victory showed the character in the Les Bleus squad.
"We made a dodgy start to the game," added Nasri. "We were heavy-legged after a week of training and the temperature weighed down on us too.
"But we felt better as the game went on and we showed our character. It was important to win.
"At half-time the coach told us to get a quick goal and said if we could do so, we would win 3-2.
"We knew this was a warm-up game with consequences for the matches to come and we showed our mental strength."
One of the top soccer cleats for control the Nike CTR360 will not let you down on the field in tight spaces.
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The new PUMA evoSPEED 1.2 FG. A great new soccer cleat with classic look and first time for PUMA to be on the cleat.