Here you will find a blog by the director of the season 2 opener, ‘Four Months Later”. If you scroll down to the 7th paragraph, you will see this little bit:

“Nathan, who was so in control and determined as a political candidate
last year, has seemingly lost it all and has no goals. He drinks alone
in a bar and is haunted by a disfigured version of himself. Who is this
vision and what is the symbolism behind his horrible disfigurement?”

It is not Peter.