How far back is "this era" -- I'm spoiled by seeing many Red Sox rookies or young players burst onto the scene and just rock the league in their first full years: Fisk, Lynn, Rice, Clemens, Boggs, Greenwell (2nd in MVP), Nomar, Pedroia, Ellsbury, Mookie...
Some notables that took a few years of adjustments: Dwight Evans, Varitek, Youk, Bogaerts... I'd say Raffy, but he's still making adjustments today to the league's adjustments to him.