Stephens was sent off for his high challenge on Alejandro Garnacho in their 3-0 loss to United on September 14th.
The defender argued with referee Stuart Attwell which led to the filing of an extraordinary incident report.
"The defender admitted that he used abusive and insulting words towards both the match referee and fourth official around the 79th minute after he had been sent off," the FA said in a statement.
"An independent Regulatory Commission imposed these sanctions, and its written reasons can be seen below."
Stephens will miss Southampton's upcoming games against Arsenal and Leicester City. He already served a three-game suspension for the red card, missing matches against Ipswich Town, Everton and Bournemouth.
Southampton are second from bottom in the league standings on one point.