I just don't get this starting LB for Argentina at WC thing. I think some people are a bit behind the times. Like you know in 70s when there was not internet and the world was much more "divided", top leagues did not have the sway and popularity they have now, of course WC was like a football central stage and basically a showcase for football. But it's not anymore. Argentina has shit LBs that's a fact, nobody would name a one really good argentinian LB who would make it in top club. And if Rojo is the least shitiest of them all it does not make him good.
Keep in mind that Demichelis was also in Argentian team, not sure anybody would want him. Netherlands team that also had a good tournament had Vlaar at CB. Other below average players as well. And look at Brazil team, anyone thinks Paulinho or Fred are good enough. Many top national teams have shit players in them in one position or two. That just how it is.
The fact is Rojo was playing at Sporting at 24, that is more or less his level, he can probably play at teams like Everton or Stoke or any other middle table one, he can be a cover for United, although on his salary it's not wise, but as a squad player, not a starting line-up one, but like 16-17 most used players over a season, he just does not cut it.