Guys, this will only take a second of your time. Go to the link below:
http://www.imdb.com/name/nm0687443/
The above link is to the IMDb page of Jack Plotnick (Brett Partridge.)

As you can see, it will display the episodes of The Mentalist he appears in. And it shows that Jack Plotnick will make an appearance in the episode 'Red John' as Brett Partridge. I am not saying that he is Red John, but I believe he will make an appearance.