Joe Garner scored the only goal of the game at Portman Road as the Blues began the campaign with a defeat, but despite Redknapp being satisfied with the effort of his players, the veteran boss suggested that signings need to be made before the end of the transfer window.
Redknapp told BBC WM: "The players gave everything as I knew they would but we are in a situation where we are short. I looked at the bench and I had no change. I need options.
"We have tried very hard but everyone we have tried for it has not happened for whatever reason. People have asked for too much money.
"If I can get the players then I believe I can build a team that will challenge for promotion but it's a weak squad at the moment."
Birmingham are next in Championship action when they play host to Bristol City on Saturday, August 12.