The main reason I wouldn't be so worried about Jose and youth is our academy hasn't produced a top prospect in a fairly long time. We've produced okay, players like Welbeck and Cleverly but we need a Martial come out of the academy. Even with the ones we've had come through of late, (Januzaj, Pereria, Varela, Pogba types) most of them are arriving at 16. If you take Wenger's stated timeline of players at that age they have already passed a couple of the barriers to top level football, and in a sense are "ready made" players. What you are really talking about is whether as a manager he would be preventing players coming through to the first team.
Even with LVG now, how many of these players are good enough. Is anyone looking at McNair and saying he's a Varane or Laporte? Unless we have those player in the academy any coach, is just blocking the progression of players to lower level clubs. United's youth as far as the first team goes, currently consists of buying players in the final stages of their development like DDG, Martial, Shaw, Depay. United's youth problem is with the players the academy is producing not the player the team is using.
Jose would be a short term manager so the only worry over youth is would he allow the team to develop a problematic age profile by not giving enough opportunity to young players, and by young we mean of the right age nothing to do with academy. Would Jose damage a player like Martial or Depay?