This is a serious argument. Sans is quick and fast to react (Like Xander) and his power is fueled by the sins of his opponent.

Average eggy who would win

Actually
Not average
Usually I'm screwing around with something stupid

This is an actual match up I thought would be interesting, and provided real evidence tos support my claim.

A one of a kind match up here

F**k you. AstroMan wins. Sans lost to a child.

A child with the powers of a GOD.
In Undertale, the power to save and reload is canon, not to mention, you also are much more than "just a child" in Undertale. The player in the Genocide Route beats down the most powerful heroine in the underground, tears apart a robot MADE by a scientific genius to kill humans, and strike down the mighty king of the entire monster race in a single blow.

I do care.
That's precisely why I'm making this.
I like your story.
I also like Undertale .
I actually thought this would be a cool match-up.

Now, yes I do make jokes using the "who would win" format an obnoxious amount, but this is an actual structured argument with real evidence to support it. I just wanted to have a fun debate.

Okay now I just feel like shit for saying that

It's okay!
You're fine, I'm not mad at you. Like you said, you were just a little pissed. I get it bud.

Other reasons to why Sans could be a worthy contender:

Sans's base stats are seemingly very weak, pair that with his lazy attitude and appearance, and with AstroMan's high opinion of himself, and thus AstroMan wouldn't expect Sans to beat the shit out of him, and that's exactly why he would.

Like I was saying in the title (couldn't fit it all), the sins the opponent has committed boost Sans' strength, and AstroMan has mercilessly murdered probably hundreds of people, which would make Sans' powers multitudes more effective than it is in Undertale.

This one is kind of stupid, but I also just want to see AstroMan get his ass kicked by someone who he thinks will be a pushover (again)