Andy Cohen -- I'll Be Lindsay Lohan's Sober Buddy ... For One Night

Andy Cohen will be earning his one-day sober chip this week because TMZ has learned he will not partake in alcohol during Thursday night's "Watch What Happens Live" ... as a show of support to his guest ... Lindsay Lohan.

For those non-"WWHL" fans ... the show always features a bartender, and cocktails are served throughout the show to both the guests and the audience ... and, of course, to Andy.

But according to our sources, Andy has vowed to not dabble in the libations for the evening, so Lindsay isn't tempted. It's unclear if there will be an open bar for everyone else, or if it will be a teetotalling night for all.

Maybe he'll intro himself. Hi, my name is Andy, and ...