The 20-year-old defender broke into the first team at the end of September as a substitute for captain Mark Jackson, he was elevated to the starting XI for the following game against Southend United and has held his place for every game since.
He has earned rave reviews and the club have moved quickly to secure the signature of the player manager Brian Laws described as the best defensive prospect the club have had while he has been in charge.
Butler said: "I'm enjoying playing my football here. I'm learning new things off Russ Wilcox and the gaffer every week. I think I'm growing in stature every game."
Butler will partner former Ireland U21 captain Cliff Byrne in the centre of United's defence for tomorrow's Division Three trip to Yeovil Town.