Naruto's battle power was never in equivalence to his rank...I mean how many gennin's have you seen that could actually have a summon, know a jutsu as hard as Rasengan, have been trained by a Sannin, are able to do a forbidden jutsu and are a jinchuriki to boot. More importantly i think Kakashi, Yamato and Tsunade all had faith in Naruto's abilities. I mean they were closely observing his growth and the technique Naruto was creating, they knew how tough and OP it would be once it was mastered. These all things combined made him a candidate to be sent to fight a strong opponent