According to Keith Law, Harper has a torn ligament in his thumb after sliding head-first* into third base on Friday night. He will undergo surgery on Tuesday and is not expected back until early July. My guess is that the Nationals will probably play this one carefully and won't rush him back until after the All-Star Game, which is July 15.
*STOP SLIDING INTO BASES HEAD-FIRST, BRAVES
This is obviously a huge blow to the Nationals, who came into Monday already four games back of the Braves in the standings. Look for Nate McLouth to probably get the majority of Harper's at-bats unless the team makes a move. Washington is already dealing with injuries to Wilson Ramos, Doug Fister and Ryan Zimmerman.
For more on the injury, be sure to check out Federal Baseball.