Spurs looked certain to qualify for next season's Champions League until February but are now in a shootout with Arsenal, Newcastle and Chelsea for third and fourth place going into the final week of the campaign.
And Redknapp revealed his belief on Monday that the club needs to spend big in order to avoid a similar scenario in a year's time.
He said: "We're a good side. If we're bold in the summer, we can build a team again that can be right there again next year. The league is changing and a top-four position is very difficult to achieve."
Tottenham are a point behind third-placed Arsenal going into Sunday's final game of the season against Fulham.
Only a top-three finish will guarantee Champions League football, with fourth coming into play providing Chelsea do not win this month's final against Bayern Munich.
It is a nailbiting end to what has been a rollercoaster season for Spurs and Redknapp, who was cleared of tax evasion in February and made favourite for the England job before it went to Roy Hodgson last week.
Redknapp said: "I'll be glad when next week is over. It's been a long old season.
"I just want to get in the Champions League and have a break and get away for a few days.
"It has been a hard year for me personally. What I went through with the court case was the most difficult thing I've ever faced in my life."