In a brief exhortation followed by the name of a sports team, such as "Let's go, Dodgers!" or "Go, Phillies!" is it ever appropriate to omit the comma?
In English, the vocative case is designated by separating the vocative noun with a comma. For example:
"I don't know, John."
"Out of the way, peasants!"
"Jacob, will you get that for me?"
Because the exhortation Let's go, Dodgers! is using the vocative case, a comma is necessary.