West Ham boss Allardyce explains Johnson signing
"All our other efforts in securing a central defender prior to knowing Roger was available were difficult," he said.
"The one big thing about Roger is his Premier League experience at Birmingham City. His height, which has been a weak point for us recently, and a desire to come and show he has still got the capabilities of coming to play at this level.
"He is not cup-tied, so going into one of the biggest games of the season for us with someone with his experience against one of the biggest sides in the Premier League is a help.
"If you look across the board at Man City, aerially we have to be very competent and Roger will be a great help in that area particularly.
"He is very determined and excited about the opportunity he has got after being basically frozen out at Wolves."