Lehtonen is probably the best KHL D-man to come over to the NHL, without being drafted first. Comparing him to Kivihalme, Oz, Zaitsev, certainly is pointless, as none were near as good, or as heavily recruited. He was heavily recruited, and we were very lucky to get him. There is little wonder, as to why some fans, are so excited about his potential. Heck, I'm excited about his potential..
But.... and it's a big one....
He's never played in the NHL, nor on the small ice, so we really have no idea at all, how he will translate, to a faster, harder game. His NHL game is an unknown, so it isn't unreasonable to say so. But, the same could have been said about Matthews, Marner etc... and every other prospect too. There are very valid reasons to be excited about him, and very valid reasons to also be cautious in projecting what he will become.
But, we won out over other teams, to bring in the best Defender, outside the NHL, without a draft pick, or any cost in a trade... and that's pretty good.