Defoe, 28, recently said he was unhappy with the lack of first-team football he had received this season, and would have to consider his future at the club. But Redknapp believes there is a simple explanation behind the England forward's drop down the Spurs pecking order - warning the 28-year-old he needs to raise his game.
"Jermain has to let his football do the talking. He is a fantastic player but his form this year is nowhere near what it has been in the past," Redknapp said. "When he is at his best, he is a fantastic goalscorer, but he has not played often this year because he has not played well enough.
"He trains well, but apart from at Wolves, where he scored two goals, he's rarely shown the type of form he is capable of. He is an 18-20 goals-a-season man, not a four-goals-a-season man.
"Rafael van der Vaart is a second striker and a link player, but he has still managed to be our leading scorer and way in front of everyone else."
Defoe also drew some veiled criticism from his manager after suggesting that qualifying for the Europa League would be a distraction rather than a bonus next season.
"I'll give him Thursdays off, maybe he's busy on Thursday nights," Redknapp joked. "We'll have to organise ourselves around that one. If he can make it, we would love him to play."
After failing to qualify for the Champions League for a second season, Redknapp believes the likes of Luka Modric and Gareth Bale will not follow Defoe's example and become unsettled as long as the club continues to show ambition in the transfer market.
"They're under contract and will only want to move if we don't show any ambition and fall away over the next couple of years," said Redknapp. "If we keep progressing, they couldn't do much better.
"They could take us into the Champions League next season. I don't think they came to Tottenham expecting to be in the Champions League every year."