Glenn Roeder has agreed to extend his stay beyond a month - a spell which ends when he plays against Leicester on Saturday - and he explains:
"Everyone at Millwall is delighted with him and I have been over there to see him play.
"Our feeling is that Mark McGhee is very happy with him, the supporters have accepted him, and that is because of his ability and the performances he has put in for Millwall.
"He is progressing very nicely and we will leave him there for a maximum of another two months, which will take him to the full three months - the most we can leave him there before we call him back.
"We are now in the situation of instant recall which means we can get him back with a phone call and he will be back training the next day.
"All the while he stays playing in the first division with Millwall he is getting invaluable experience at a very high level.
"He is just 18, and if he can put down a dozen or more appearances for them that is going to set him up nicely for the second half of the season for us.
"That will put him in a position where we hopefully be able to blood him in the second half of the season, whether it be a start or coming on from the bench.
"But you would like to think that Glen Johnson is now going to materialise into the sort of player that we always felt he could."
Reports had suggested that he would be on loan on a monthly basis, allowing West Ham instant recall, until May but that was never the case as it is not possible within the rules.
Equally, there was never going to be a chance of him going on a season-long loan as that does not allow for the recall option.
"He won't be staying there longer than three months and hopefully he will get the full three months in and there won't be a need to call him back," adds Glenn.
"If he gets into double figures it will be a great start to his career.
"We mustn't put too much pressure on him - but we couldn't be any more delighted with the progress he has made since joining Millwall, and Mark McGhee is very grateful that we have loaned Glen to him.
"We have returned that complement in that we are very happy for Mark to play him in the team - and he is in there purely on merit."
Glen is one of three players out on a temporary basis, and Glenn adds:
"We have Grant McCann at Cheltenham and Izzie Iriekpen at Leyton Orient and I suppose there is a possibility of Leon Britton might be asked to go to Southend on loan after a couple of reserve games over there.
"If they do ask I won't stand in his way as it would be a great experience for him.
"Other than that, the squad is not big enough to loan too many others out."