Pryce, 19, was dismissed by referee Liam Moore just after the hour mark for a dangerous tackle on Connor Wynne.
Watson had no argument with the red card the eighth in Super League this season – and insisted: “It is a big lesson for him to learn.
“He is devastated in the changing room.
“He knows he has messed up, what he has got to do now is lick his wounds and see what consequences come on the back of those actions and then learn from them.”
Pryce is likely to face a suspension when the RFL’s match review panel meet today and Watson added: “He’s a good enough kid to understand and learn from it.
“You go through some adversity in your career and this is one Will has got to face.”
Watson admitted his team’s ineffective attacking performance cost them the game.
They trailed 6-0 at the break and were 14-0 down when Pryce was dismissed.
“I thought the effort areas were outstanding,” said Watson.
“I thought we worked really hard for each other, but our attack wasn’t clinical enough, especially in the first half.
“In the second half we made some little breaks and dented the line, but it just seemed not to be happening for us.”
“At half-time we said, in the first half we were off slightly in terms of how we defend and definitely in the way we attack.
“When we did get attacking positions our attacking play wasn’t good enough.”