Conor McGregor wasn't just up and on his feet a week after ACL surgery ... the UFC superstar was filming an ad for his MAC Energy drink, and we got a BTS look!

Warning: If you're Conor's doctor, close your eyes.

The commercial, which was filmed last week in California's San Fernando Valley, shows The Notorious on a military-inspired set, rockin' full tactical gear, for a scene straight out of an Arnold action movie.

It was only eight days after McGregor went under the knife for his right leg injury ... which he sustained during the opening moments of his comeback fight against Max Holloway at UFC 329.

Joining McGregor on the shoot? Internet star and fitness influencer Ashton Hall.

Conor posted the final version of the ad, captioning the post, "8 days post surgery. Doctor said rest. I said load the fkin belt. This is MAC ENERGY," tagging Hall, welcoming him to the "MAC movement!”

Speaking of MAC, the former UFC champ-champ launched the energy drink in July, and it's blown up ... and it continues to grow at a rapid pace.

Most athletes would still be stuck in bed with an ice pack and elevated leg ... but not Conor!