The England international scored twice in the Bluebirds' shock win over Manchester City on Sunday to give his side their first win of the season.
Redknapp wrote in his column in the Daily Mail: "On Sunday - in front of a lively home crowd - he terrorised Manchester City and scored twice.
"The former Manchester United striker has been around a bit on loan and has suffered from terrible injuries.
"Now if he can stay fit (and he looks strong and seems to have bulked up a bit), he is critical for Cardiff. I hope he can carry on playing like that because there is a lack of English strikers around."
Cardiff face Everton in their next Premier League game next Saturday.