Parker - who had been tracked by many top-flight clubs throughout the summer months - eventually completed his move to White Hart Lane at the end of August for a fee believed to be in the region of £5.5 million.
Redknapp conceded that many people at the club were wary about signing Parker due to his age and contract demands.
However, the Spurs manager consequently thanked chairman Daniel Levy for showing his trust in him by allowing him to sign the England midfielder.
"The hardest signing for me was Scott Parker," Redknapp said, as reported by the Daily Telegraph.
"It wasn't a deal that the club were crazy about, and I could see their point of view.
"Scott had a four-year contract at West Ham and he was on big money, by our standards. Tottenham don't pay the wages that Chelsea or Manchester City do, so there would be quite a gap.
"There was a feeling that bringing in Scott, 31 years of age and with no sell-on value, could be a problem. I argued that it was what he could give us this year and next that could make the difference. Daniel [Levy] backed me on that."
Redknapp, who is widely-tipped to succeed Fabio Capello as England manager, feels that the signing of Parker is integral to Tottenham enjoying a successful season.
"I felt there was a bad feeling around the club," added Redknapp. "You know when things are not going right: Luka [Modric] wanting to go, three or four other players not happy, not pulling their weight in training, disrupting a little bit.
"We looked very, very lightweight. It was a team that scared me a little bit. I just knew we needed to get Scott Parker into the team, get Luka's mind right, and then we would be a different team."
By Elliott Binks
