For every Jason Heyward that begins his rookie year right from Opening Day, there is a Buster Posey who is called up in the summer to delay his major league service clock.

These two rookies played an equally significant role for his respective team in 2010, though they reached the majors at different times.

Midsummer call-ups can alter a pennant race and a team’s fortune for that season, and 2011 won’t be different.

There are several talented rookies that will start 2011 in the minors, whether it’s to delay their service clock or tweak something minor in their game.  However, come June, many of them will start to be called up.

Here are five players who will be called up this summer who will have the biggest impact.

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