Fagerson, the Scotland prop, was sent off for dangerous play when he charged into a ruck and caught the head of Wales’ Wyn Jones with his shoulder.
In the aftermath of the game, which Scotland lost 25-24, Watson described referee Matthew Carley’s decision as a “rubbish call”.
The Scotland flanker said Fagerson’s ruck clear-out did not even merit a penalty, let alone a red card.
“We as players strongly disagree with that call,” said Watson. “As soon as I saw it on the TV, I was thinking, ‘That’s not even a penalty’.
“We don’t think it’s a red, a yellow or even a penalty. But that’s the way the game is going.”
However, having had time to reflect on the incident, Watson has issued a more measured response.
Replying to a video of his original remarks posted by STV, the Edinburgh player tweeted: “Poor comments from me, emotions were very high after losing a very tight match. Players health is paramount.”
Fagerson, meanwhile, will face a disciplinary hearing on Tuesday evening via video conference.
The prop is accused of infringing Law 9.20 (a) & (b) which states: “A player must not charge into a ruck or maul. Charging includes any contact made without binding onto another player in the ruck or maul. A player must not make contact with an opponent above the line of the shoulders.”