Formalities on an initial loan deal, but with a permanent transfer to follow soon, were completed late on Wednesday morning, and Alan says:
"I thought it was important that we boosted the squad quickly, and I have captured a player that I feel is going to add significantly to us.
"The beauty of Hayden is that he can play in various positions and give us cover in a number of those, though unfortunately, that has held back his progress in the last couple of years.
"He is 24 and has played 250-odd first division or Premier League games - and in my opinion this boy is a Premier League player.
"But his immediate purpose for us is coming here now and I am sure he is going to play every week; he might be jumping about for a little while until we get our side nailed down, and his value is versatility as much as his ability."
As to where he might settle eventually, Alan says:
"I think ultimately he is a central midfield player for us and I think his energy will be important; he will have to battle in the midfield.
"The number 10 shirt is free of course, but Hayden is not a number 10 by any means.
"He is someone we are going to look to when we are up against it, to win the battle and give the room to people like Don Hutchison, Michael Carrick, Rob Lee, and Kevin Horlock to play.
"He is very much a destroyer and I think we are desperate for one of those."
Various top flight clubs such as Portsmouth and Birmingham had been rumoured to be tracking his progress, and Alan adds:
"I think, to be fair, Premier League clubs have looked at him for a long time and not gone for him; I feel that is as much his fault as them not taking the gamble, and he needs to get another 20% out of his game - I see that is in there, and it is up to me to get that out of him.
"He has got to be a better player at West Ham than he was at Palace - I think he can be, and I have no doubt he can cope at the top level.
"He is so quick and so good defensively that you could actually give a job to do on somebody in the Premier League and I don't think you would see them.
"I think that tool would be important to us in our armoury, but let's not start thinking about the Premiership yet because we have to remember where we are - this player has got to help us get out of this division; that is the primary aim."
An initial loan move was agreed simply to make him eligible for the Nottingham Forest game, but there should be no complications about a permanent move.
"I have made sure he is available tonight; with a lot of players I wouldn't consider them after a couple of days thinking about and negotiating the move - it is quite a stressful time for a player," explains Alan.
"But I think he is one of the few players that has got the character to handle it - he is a pretty cool customer and I think he will not worry if he is called upon to come into the side.
"After looking at the games on video and being there Saturday I do feel we deteriorate as a team at the end of the game; we need some energy, and we need someone to cope with the Rotherhams and Gillinghams we are going to get.
"Physically he can cope with that and, as I said, there are some great qualities in this squad but the attributes he brings are not in abundance - and we are going to need them."
Hayden will wear the number 17 shirt for West Ham.