The youngster's latest setback will see him miss out on a place in the England squad for this summer's World Cup in Brazil.
"It's a tough one to take for him," King told BBC Sport. "One thing I would say is that he is say young enough to come back again.
"Right now it is important for him to get himself right, to get himself fit and bounce back for next season. This setback will make him stronger mentally but it is vital he recovers well and gets himself back for pre-season now."
King was forced to retire in 2012 following a series of knee problems.