Turning the U.S. men’s national team into a relevant soccer country was one thing. But Mauricio Pochettino, the coach who guided the Americans through their home World Cup, is leaving for an even more daunting reclamation project—at one of the biggest clubs on the planet. Next week, he will quit the U.S. to become manager of Manchester United, the ailing English giant off to the worst start in decades. Pochettino’s reward for taking on one of the most prestigious jobs in soccer is an immediate relegation battle. United, which has just one victory in 10 Premier League games this season,…