Glen has joined the Lions for a four week period with his boss insisting: "When I spoke to Mark McGhee about the possibility of him going to play in the first division I felt it was a great opportunity for him.
"He lives 20 minutes from the training ground with his mother and has three or four school friends that are on the books there, so he is going into an environment he knows.
"There were two or three second division clubs that have been asking about him for a couple of weeks now and, had the opportunity not come up to join Millwall in the first division, I would have allowed him to go the second division for a month."
While one or two less well informed fans have grumbled about a deal between the two clubs on account of their perceived rivalry, Glenn stresses: "My only consideration is the player - and always will be.
"I have got no other agenda and when I look at everything I feel that Glen will be a first team player here in the future."
It is of no consequence to his opposite number at the Den either, and Glenn says: "Funnily enough we never discussed it; with two London clubs we know there is rivalry but surely the only thing that matters is Glen Johnson's football career - and this is part of his learning curve.
"We have real belief in him and now he has got to go out and perform; we feel he will be up to it and when he comes back after a short period of time it will be a job well done.
"We don't want to loan him out for ever and ever; the initial agreement is for a month and if he can get four games in that will have broken the ice for him as far as league football is concerned.
"We think it is a good idea to blood him in the football league at the highest possible level to see how he copes; his first game is at Norwich on Saturday and that is going to be a tough game for Millwall - and a tough game for him.
"But hopefully that will bring the best out in him.
"The following week they are at Derby and it is a wonderful experience for him to be playing just one division down from the Premiership.
"We will have every game covered with a detailed report from our scouts and I am sure Mark McGhee will keep in touch with me.
"I am very comfortable with the manager he is going to, and the coach, Ray Harford who is over there; they both have excellent reputations.
"I know from talking to people that work with Ray that he is a very good coach and when he comes back, which will probably be in a month, he will have moved on a level or two; Ray and Mark will do him good and that is why I have allowed him to go there."
Another loan deal could be concluded in the not too distant future and Glenn adds:
"Although there is a transfer window we know we can loan these players out for a month at a time as we have done with Grant McCann at Cheltenham.
"Izzie Iriekpen is available and we are having enquiries about him - I shall allow that to get him league experience."
That move, Glenn feels, will have a beneficial effect on some of the even younger players at the club and he explains:
"By moving Izzie on it will give an opportunity to lads like Elliot Ward, who had an excellent game against Tottenham, playing against Les Ferdinand for the 45 minutes he was on.
"He is only a second year, and when Izzie goes out for a period of time on loan we can get Anton Ferdinand into the team alongside Elliot.
"And we couldn't have a better goalkeeper for young players to play in front of - Raimond is superb and his professionalism can only rub off on the young players.
"If the reserve pairing is Ferdinand and Ward, in front of Raimond, it can only move these young boys on quicker, and that is what we need to do.
"We are looking to fast track them if we can - but not to the point of taking any chances if they are not ready.
Glenn is not against the loan system although he uses it carefully, and he hopes that the move to Millwall can be the springboard for a Premiership debut before the end of the season.
"Glen is not ready for the Premiership yet but we hope that by him going to Millwall we will gain even more confidence about him before the end of the season pushing for a game or two in the first team," he says.
"If he does that will be wonderful for an 18 year old for whom we have high hopes.
"Rio Ferdinand, Frank Lampard and Michael Carrick in recent year have gone to Bournemouth, Swansea and Swindon respectively - and come back better for the experience.
"I am sure this will be the same for Glen, Izzie, and Grant.
"When I am looking at clubs to let the players go to my only concern is getting them to the highest level that I think they can cope with, and who the manager and coach is."