Defoe, who scored his first Premier League goals of the season against Wolves at the weekend, is often the man to miss out when Redknapp switches to his preferred option of playing Dutch forward Rafael van Der Vaart behind a lone striker, usually Peter Crouch, for key clashes.
"It's been hard. You start to think: 'What am I doing here?' I'm used to playing in big games," Defoe told the Daily Mail. "I've played in big games for my clubs and for my country."
And Defoe admits he is worried he will drop to the bench again for the Wednesday night's Champions League last-16 second leg against AC Milan, despite scoring twice on Sunday.
"It'll be a tough game (against Milan). I hope I'm playing and if I can score and we win the game then it'll be great. But I remember playing against Charlton and scoring two goals (in a 3-0 FA Cup win in January), then the next weekend I missed out.
"I was upset about that because once I start scoring I'm confident and I have the feeling I know I'll score in the next game. We played Manchester United and I thought, 'I'm playing', but I didn't start."
Leaving in the transfer window did cross Defoe's mind. "I don't think the club would have allowed me to go on loan," he said. "But it was like when I decided to leave here and went to Portsmouth because I got to a situation where I thought: 'I'm not sitting on the bench.'
"I'm 28, I don't need to be sitting on the bench. I don't need anyone to put their arm round my shoulder and say: 'Just keep it going.' And if it means I have to work even harder, then I will."