The Manchester United wonderkid didn’t do anything too spectacular during his fourth appearance for the national team last night. But that’s not really the point. Alejandro Garnacho, playing the entire second half as Argentina defeated El Salvador 3-0 in a friendly, kept things simple. The odd line-breaking dribble here, a well-timed pass there. At the age of only 19, however, the excitement that greeted Garnacho’s 45-minute outing was not because he burst the net with a Puskas contender like at Goodison Park. Not because he ran the defence ragged like he did against Liverpool in the FA Cup qu…