I think Matthew Tkachuk, in particular, will be difficult to contain down below the goal line. My view is that Parayko was a really solid pick by the Canadian management team as he has the skating ability, size and strength (and the skill) to matchup with Matthew down low and provide a good level of defensive cover and physicality.
For the U.S. team, as a Canadian fan I'm more worried about Matt Boldy than I am of Brady Tkachuk. Boldy is more versatile as a player and provides a somewhat similar skillset as Matthew Tkachuk. Depending on how the Americans line up their forwards, I think a M Tkachuk/Matthews line, followed by an Eichel/Boldy line (or however they line up) will be challenging for Canada. Toews is a good all-around d-man and Doughty is one of (if not) the best 2-way d-men of his generation, but without Pietrangelo the Canadian D lacks size, strength and physicality.
I don't worry so much about either of the Tkachuks' abilities to get under the skin of their opponents. Canada is full of experienced professionals, and if it comes down to it two can play at that game (Bennett, Konecny, Marchand). The bigger issue is containing the strength and physicality of the American forwards imo.