Manchester United defender Mikael Silvestre has been taken to hospital with a suspected dislocated shoulder.
United booked their place in the quarter-finals of the UEFA Champions League with a 1-0 victory over Lille but Silvestre was injured in stoppage time as he went up for an aerial challenge and landed awkwardly.
Play was held up for several minutes until the Frenchman was stretchered off clutching his shoulder.
And Ferguson admits the injury could be extremely severe.
"We fear the worst and we don't know whether it's broken or a dislocation, but he's on his way to hospital," he told Sky Sports.
"We'll just wait to see if it's anything really serious."