Walker, capped four times by England, said: "I know I have the rest of the season here. Beyond that I don't know how things are going to pan out.
"Jussi is having such a good season, and the side is doing very well. I have to accept that is the situation.
"If someone is doing that well you can't say a great deal about it.
"I push myself as hard as I can in training, so I am ready whenever the gaffer [Sam Allardyce] wants me. I will just keep plugging away and see what comes up."