Ultimately, these are just exhibition games. I'm not so sure the 'rulebook' on this tourney was thorough enough to capture or dictate this type of circumstance. From the beginning, it was touted as a 'best on best' competition. Really, it would be kinda silly if they enforced strict rules against fielding the best team - especially at the expense of regular season games.
If they ever do this again, they should implement a deeper roster to cover injury and illness. Last night proved that - US only dressed 11 F, 6 D to START the game. A deeper player pool would've covered that and allowed Quinn Hughes to be included on the US roster as a possible replacement with no questions asked.