Guillard showed great takedown wrestling defense and striking early in the fight, but Miller was able to take Guillard's back and pull off the submission.
Guillard has now lost his second straight fight by rear-naked choke.
Aside from the two straight losses, Guillard recently left Greg Jackson's camp in order to train with the Blackzilians.
Perhaps this was not the best time for Guillard to change camps, especially since he held Jackson in such high regard.
Despite being one of the most experienced and athletic lightweights in MMA today, Guillard has often struggled in big fights and against submission experts.
What Guillard needs to focus on when he goes back to training is his submission defense.
Nine of his 10 losses have come by submission.
Guillard may also need to refocus himself so he can come back strong after two straight losses.
As far as future opponents go, Guillard could take on former teammates Donald Cerrone and Clay Guida who are both coming off of losses.
Fights with Sam Stout, who also lost in his last bout, and a returning Sean Sherk could also be good fights for Guillard.