Redknapp swooped for Kranjcar last August from Portsmouth.
He said: "When you talk about signings of the year - Niko is right up there.
"I was the manager at Fratton Park who brought Niko over for the same money from Hajduk Split in Croatia to the Premier League.
"He is a terrific player with a great touch and solid technique. The way he has played for us since signing in September has been top class.
"It was a while before he got a run in the side but now he looks unshakeable. I’ve developed good contacts with people in Croatia like their national manager Slaven Bilic, Davor Suker and Igor Stimac.
"A bit of a bonus is that Niko has been weighing in with some goals from midfield. He's got eight this season so far - seven for us.
"And, of course, there is the great understanding he has with his Croatian team-mates Modric and Vedran Corluka."