Glasgow's Scotland flanker Ryan Wilson has been cited for allegedly grabbing the testicles of Northampton's scrum-half Lee Dickson.
If found guilty of the incident, which occurred in the 25th minute of Sunday's European Champions Cup game, Wilson, 26, could face a minimum 12-week ban.
Wilson's disciplinary hearing will take place on Wednesday, with any ban likely to rule him out of the Six Nations.
Glasgow lock Tim Swinson has also been cited after a yellow card for punching.
Swinson was sin-binned in the 74th minute, but his offence could bring further punishment after being highlighted by match citing commissioner Jeff Mark.
Wilson has won 15 Scotland caps since his debut against Wales in 2013, and featured in four games at last year's Rugby World Cup.
He is likely to be included in head coach Vern Cotter's squad for the forthcoming Six Nations, which will be announced on Tuesday, pending any disciplinary proceedings.
Under World Rugby guidelines, the offence of "testicle grabbing or twisting or squeezing" carries a suggested punishment of 12 weeks at the low end of the scale, to 18 weeks for a mid-range offence, and anything from 24 to 208 weeks (four years) at the top end of the scale.
Wilson missed last year's tournament after being suspended for three months without pay by Scottish Rugby following a conviction for assaulting a fellow rugby player on a night out.