De Gea was signed from Atletico Madrid in 2011.
"He missed a Manchester derby in 2000 for his son's wedding. The other time? To scout the brilliant David de Gea.
"I joined United in 2008 and, from that moment, we needed to be ready for the day Edwin Van Der Sar retired.
"I was convinced that a 19-year-old De Gea was the right man to replace him. I showed a three-minute DVD compilation to Sir Alex and then we went to watch him play for Atletico Madrid at Valencia on the night United played Scunthorpe in the League Cup.
"Sir Alex knew he was the right signing inside 65 minutes. David showed composure, concentration and reflexes. He made one terrific save that went through a load of bodies."