Horsfield is accused of using abusive and/or insulting language towards linesman Phil Sharp, who advised match referee Rob Styles to issue a straight red card.
The referee missed the off-the-ball incident involving Horsfield and Mark Hudson, but Sharp had a clear view of the 'clash' and television replays appeared to show Horsfield swinging his arm.
Horsfield has until 6pm this evening to respond to the charge, and United will make their submissions tomorrow ahead of a disciplinary hearing next week.