You can't overstate this. When LeBron left Cleveland, the team went from 61-21 to 19-63. 19 and 63! When Jordan left the Bulls, they went from 57-25 to 55-27.
The Bulls had a great mix of elite players (Pippen and Rodman) and nice role players (Grant, Paxson/Kerr, Kukoc) to complement Jordan. So when Jordan left, that Bulls team was still a playoff-caliber squad. Cleveland was nothing of the sort without LeBron.