Fitzpatrick, 29, a former Widnes player, was sent off on 29 minutes after throwing a punch at Vikings hooker Thomas Coyle, who was also dismissed.
The incident occurred after Coyle caught Fitzpatrick with a high tackle as the full-back retrieved the ball following a kick down field.
Referee James Child sent off the pair and while walking to the tunnel, Fitzpatrick threw his sports drink in to a section of the home crowd at the Stobart Stadium.
A club statement said that Fitzpatrick had no recollection of the event and was suffering from a severe blow to the head.
Salford Director of Football Steve Simms said: “Karl was shook up by our version of events and he has no recollection of any of it. The blow to the head has knocked the sense out of him and I guess we should be thankful he is not more seriously injured.
“However, none of this condones what happened out there, for which Karl and the Club are deeply apologetic. The incident took the shine off a good performance from us and I hope the Widnes fans can accept Fitzy’s apology and understand that he is certainly not a malicious player, and is usually exemplary in the way he conducts himself.”
Salford comfortably won the game 36-0, with Fitzpatrick scoring a brace of tries before his dismissal. Despite the two red cards, both sides returned to 13 men after the half time interval.