The Croatian midfielder made the move to Loftus Road on transfer deadline day, joining on a season-long deal from Dynamo Kiev.
It is the third time in his career that he has played under Redknapp, having previously spent time with the manager at Portsmouth and Tottenham Hotspur.
He has played three times so far this season and Redknapp expects him to go from strength to strength as the months go on.
Redknapp would then be keen to strike a permanent deal, but he is unsure whether Dynamo would be prepared to let him go after spending a reported £7million to recruit him from Spurs in 2012.
"Niko's a good player," Redknapp is quoted by the Fulham Chronicle. "He's got 70-odd caps for Croatia, who have produced some fantastic players over the years.
"He's a really good player and will get fitter with games. I took him to Portsmouth and Tottenham and I'm pleased to have him here now. He's a smashing fella, a great lad.
"You would love to have him here [permanently], but the end of the season is a long way off, so let's see what happens this year at QPR.
"I don't know exactly what the situation is. They paid a lot of money for him. We'll have to see what happens."