Johnson could be back in the first-team squad in the next couple of weeks after playing in a reserve team match in midweek, but Redknapp has insisted that Chevanton will remain a key part of the team.
The former Tottenham Hotspur boss told reporters: "I've only got AJ and Charlie Austin. Bobby Zamora is still not fit.
"If he keeps doing okay in training and when he finally gets a chance to play, I am sure that he'll do okay.
"He's been unlucky because we haven't been behind - not that I want to - but if we do, then he is one that you would want to throw on."
Chevanton has made just one substitute appearance for the Championship club since signing a short-term contract last month.