Youssef, signed in the summer, scored for the reserves in last week's 1-0 win over Derby County and Glenn says: "It was a very young reserve team that won 1-0 against the league leaders Derby at Dagenham and Redbridge, who have been very good to us this year and have always provided a pitch that is in excellent order.
"I was just pleased to see Sofiane playing, but it is just unfortunate that he slightly tweaked his hamstring and he had to come off just after the start of the second half.
"But he is still a player who has got immense potential and for someone who is 18 he is too quick for his own body, I think, at the moment - which is why he keeps picking up muscle injuries.
"When he finally stops growing, strengthens up, and matures, I still think we are going to have an exciting talent on our hands."
Glenn had predicted him having a season not dissimilar to Jermain Defoe's last season, coming off the bench and scoring goals.
It hasn't happened as he has missed so much of the campaign through injury, and Glenn admits: "I was anticipating him being fit more; if he had he would have played more - and I am sure he would have progressed more than he has done.
"But the one thing he has done since he has been with us is grown, and he is going to be a lovely size for a striker, approaching six foot and very much on the lines of a terrific athlete.
"He is lightning, really, really quick, and very positive when he receives the ball; it can be quite terrifying for central defenders to play against that sort of pace.
"He is not 19 until July but as long as we send him away fit and healthy at the end of this season for Youssef Sofiane it is all about next year now."
A young side, with the exception of Christian Dailly and Gary Breen, take on Ipswich Town on Tuesday evening and Glenn adds: "I feel it is so much better when you have young players in the reserves; they are really motivated and still have lots and lots to prove as they are on an upward curve as far as they are concerned."