Kjell Jonevret was in charge of the 22-year-old for three years at Molde, before the Senegalese joined Manchester United earlier this month.
Diouf scored one of the goals in United’s 3-0 victory over Burnley on Saturday within minutes of coming off the bench.
Jonevret was quickly made aware of Diouf’s goal.
He told Norwegian newspaper Verdens Gang: “It was hard for me to miss. I think I’ve got 40 text messages.
“I saw him training with the United squad a few weeks ago and it was exciting to see him with the big boys. Mame went in and showed that he was ready to take the battle.
“I’d heard they appreciated the way he had made his mark on the squad but I was a little bit surprised to see him playing for the first team so quickly.
“Mame is a little different kind of player to the likes of Berbatov and the others. United have many strikers that meet the ball, whereas Diouf runs behind the defenders like a shot out of a cannon.
“To come in and score at this level is almost unbelievable and it is inspiring for all of us at Molde.
“I don’t think Unityed expected him to be a key player in 2010, more a long-term prospect.
“If he continues to take steps forward he will be starting games soon.”