Krishna says that even he and Arjuna cannot defeat Karna if Karna had his kavacha (arnour) and kundala (ear rings).
Source from Mahabharata Drona Parva
Clad in (natural) coat of mail and decked with his (natural) ear-rings, Karna, who had his senses under control, could singly vanquish the three worlds with the very gods. Neither Vasava, nor Varuna the lord of the waters, nor Yama, could venture to approach him. Indeed, if that bull among men had his armour and ear-rings, neither thyself, bending the Gandiva, nor myself, uplifting my discus, called Sudarsana, could vanquish him in battle. For thy good, Karna was divested of his ear-rings by Sakra with the help of an illusion. Similarly was that subjugator of hostile towns deprived of his (natural) armour.