His game shape likely has deteriorated and will need a week+ of minutes to get back to just where he was. The game assumes no practice or training while injured. What's even more strange is a player's game shape drops after you purchase him from the market.
Game shape drops after a sell/purchase is alright, it's a proper way of dealing with daytrading and other suspicious market practices.
But... if there was a way of making injuries fair and realistic, they should be definitely linked to game shape... higher levels of game shape should prevent or stop injuries (level 9 should never lead to injury, level 8 extremely rarely, level 7 less rarely and below 6 injuries should happen often)... mastering game shape is hard and should be rewarded, this would be fair and realistic, way more than the random crap which has always been injuries in BB, probably the worst (of the very few bad features) feature of the game.