Redknapp replaced Mark Hughes at Loftus Road in November with QPR bottom of the table on just four points.
Rangers were relegated to the Championship along with Reading following the 0-0 draw between the teams on Sunday.
The QPR manager spent over £20m on bringing striker Loic Remy and defender Christopher Samba to Loftus Road in January, but Redknapp reckons he has been battling against the odds from the start of his reign.
"If you had the job at the start of the year it would have been different because you build your team and that is your team," Redknapp said.
"You shouldn't be running around in the transfer window trying to patch a team up, it is very difficult then. I spent days and days away from home all over Europe trying to plug some holes.
"You build your team in the summer. That is when you build a team, do your training and get together. The club went 13 games without winning a game and that is quite a bad start by anyone's standards."