Manchester United boss Sir Alex Ferguson is 'frightened' by the potential of Barcelona star Lionel Messi.

Ferguson has warned it is "frightening" that Messi can only get better.

The Manchester United manager said: "Players develop and start to ­mature when they reach 25 or so. We've seen that with Cristiano Ronaldo, Wayne Rooney and ­others. But Messi has not even reached that age yet. It's frightening what's going to happen - he's only 24!"