The 28-year-old has provided several assists and formed a strong partnership with Peter Crouch, but he has seemed off the pace recently - partly due to a calf injury.
"His fitness is a problem for him. He tires in games at times and you pick up injuries when you're tired," Redknapp said.
"I take him off because I feel he is looking tired and it's my decision at the end of the day what I do. If I decide to sub him, I'll sub him.
"That's why I have played him for 60 minutes or 70 minutes. Can he play 90 minutes tomorrow and then play 90 on Tuesday? I will have to look at him. I don't want to over-do it with him and leave him struggling for Tuesday."