ARLINGTON, Texas – George Lombard Jr. is suddenly on the doorstep of the big leagues. The Yankees have promoted their top prospect to Triple-A Scranton/Wilkes-Barre, a source confirmed on Tuesday night, after he crushed the first month of the season at Double-A Somerset. Lombard, who turns 21 on June 2, was batting .312 with a .971 OPS in 20 games at Somerset after going through some growing pains in 108 games there last year. Somerset Patriots shortstop George Lombard Jr. scores during a game against the Reading Fightin’ Phils. Charles Wenzelberg / New York Post The Yankees believe that the shortstop — who has also played some occasional third base — is already ready defensively for the big leagues, but they wanted to see his bat catch up to his glove.  Lombard has impressed in each of the last two springs, as a non-roster invite to big league camp, both with his talent and his character.…