Terrible break for the young man. Not a good thing for the USMNT either, but while he has been our most consistent fullback the past year I don't know that's he truly irreplaceable from the pool we have. Between Brooks, Zimmerman, Richards, EPB, Ream, Miazga, CCV and others, I don't view this loss (and he's clearly not playing in this World Cup) as being as hard to overcome as not having McKennie would. (And despite what Subba said, I'd be really surprised if the current injury cost him the World Cup).
As for CCV, Berhalter said he was coming in this summer to get his shot before this happened. He was first team all league in Scotland, and, as said above, some have argued he was one of the best players in the league. Spurs still own his contract. Will be interesting to see if he stays with Celtic, who would like to keep him, or ends up back in England with either a lower tier Premier League team or top level Championship team.