Warren Gatland believes England prop Kyle Sinckler "emotionally, can be a bit of a time-bomb" ahead of Saturday's Guinness Six Nations showdown.
The Harlequins forward has twice been involved in incidents during this season's tournament.
In England's Six Nations opener against Ireland he became embroiled in an exchange with flanker Peter O'Mahony, and in the France game eight days later he was reminded of "rugby's values" by match referee Nigel Owens after slapping Les Bleus forward Arthur Iturria on the head.
Speaking last week about the Iturria incident, Sinckler said: "I don't condone what I did, but they didn't show the full replay of what actually conspired. I didn't just smack him on the head. There was stuff that happened before.
"I am not perfect, and I have learnt from it."
Sinckler, who was coached by Gatland on the 2017 British and Irish Lions' New Zealand tour, will start against Wales in Cardiff.