I don't think Haku was close to Kakashi in terms of strength, however Zabuza maybe (i don't rate Zabuza).

I mean Haku got killed by Kakashi's chidori, if he was so powerful he could have countered it or somthing.
Kakashi also smacked the crap out of zabuza afterwards. I know Kakashi got done by the water prison, however, i think that scenario was created as a twist and an oppenning for Naruto and Sasuke.
It wouldn't have been a great saga if Kakashi killed Zabuza that early.