When did Hotch become Unit Chief

Watched Season one and on to Season two again and wondering is there a moment where Hotch is in charge or put in charge? It seems like Gideon is set to be the Unit Chief as Morgan refers to him as that in the first Episode, but they also have the scene of another Agent wanting Hotch to evaluate Gideon and said he needs to be able to step in. Then by the end of Season Two Gideon refers to Hotch as the best Unit Chief he knows, so I’m wondering if anyone else has any idea