Lack has had a few blips but only since he's become the 'starter'. While he was the backup, he was lights out. Look at the St. Louis games. He just needs to get accustomed to the grind of playing every game at the NHL level, and he needs a better defence in front of him.
Being the actual starter is an interesting theory. There have been goalies in the past who posted strong numbers as back ups but never could replicate it as a starter.