Xavier Nady's Magnetic Resonance Imaging test yesterday revealed the Pirates' right fielder has a Grade 1 sprain in the acromioclavicular joint in his left shoulder.
There is no damage to the rotator cuff or labrum.
The Pirates will wait two or three days before making a decision on placing Nady on the disabled list.
They begin a three-game interleague series in Chicago against the White Sox tonight.
The designated hitter will be used, which means the Pirates can afford to "play short." Pinch-hitters will not be used much.
However, they begin a three-game series against Toronto at PNC Park Friday night. No designated hitter is used in National League parks in interleague play.
Nady, who will rejoin the Pirates today, injured his shoulder when he crashed into the right-field wall Saturday night while trying to catch a foul fly hit by Baltimore's Ramon Hernandez in the ninth inning.
NOTE -- Pirates catcher Ryan Doumit yesterday was named the National League's co-Player of the Week, tying for the honor with Florida pitcher Ricky Nolasco.
Doumit led the NL with four home runs last week -- all in a three-game series against Washington.