Lock Bradley Davies will miss the rest of this season's tournament after collecting a seven-week ban for a reckless tip-tackle on Ireland replacement Donnacha Ryan in Dublin last Sunday.
Davies is the second Wales player suspended for that offence this season following Sam Warburton's dismissal and three-week punishment for an incident in the World Cup semi-final against France last October.
"He (Davies) knows that he shouldn't have done it," Wales fly-half Priestland said.
"It is one of those heat of the moment things. He didn't try to do it, but looking back he knows how silly it was.
"We spoke about it as a team this week. We can't let our discipline cost us games.
"I don't think any player goes out to do it, it is just one of these things that happens in a game, and it is just important that we try to stop it happening as often as possible.
"I am not sure if it is a technical issue. I sort of tip-tackled Jonny Vaughton (ex-Ospreys wing) once, and as soon as I picked him up I knew that he had gone and what I had done, but it had already happened and I couldn't stop it."
Wales, meanwhile, are confident they can avert any threat of a second-row crisis after losing Davies following the 23-21 victory over Ireland.
Cardiff Blues forward Davies, subject to any successful appeal, cannot play again until March 26, nine days after the competition finishes. He joins his fellow World Cup locks Luke Charteris and Alun-Wyn Jones, who are both injured, on the sidelines.
The punishment would have been considerably longer had it not been for various mitigating factors, including Davies' guilty plea and good disciplinary record,
Wales coach Warren Gatland will announce his team to face Scotland next Sunday on Thursday, with former captain and back-row regular Ryan Jones widely expected to take Davies' number four shirt.
Jones, who starred at blindside flanker against Ireland, has played lock for his regional team the Ospreys this season, and would be the obvious replacement for Davies, although Gatland also has uncapped Scarlets second-row Lou Reed in the Six Nations squad.
"Ryan Jones has been on top of his game as a second-row and back-row for the Ospreys, and we are really happy with the way he is playing at the moment," Wales forwards coach Robin McBryde said, speaking prior to Davies' ban being announced.
"We have that option, even if it would not give him as much freedom to run around the field carrying the ball as he did so well on Sunday.
"And we have Lou, an up-and-coming second-row who was with us at our training camp in Poland and has plenty to offer. He does not look out of place and he is fitting in well, taking a lot on board."
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