Sounds like his knee buckled on a play behind 1B. He's a FA after the season so this could be a seriously costly injury to him if the MRI shows torn ligaments.
Not a doctor but its hard for me to believe that its not a season ending injury. Non contact knee injuries like that are almost always worst case scenario. Feel bad for the dude as he was a FA after this season.
The Phillies will be without another big bat for a while. Darick Hall, Josh Harrison, and Kody Clemens are all in camp and can play at 1st and Alec Bohm can move over from 3rd to play there with Harrison and Edmundo Sosa taking over at 3rd.
They could always take a flier on a guy like Miguel Sano who the Twins tried at first with mixed results.