Redknapp led Spurs to a first-ever Champions League campaign in 2010 and guided the club to two fourth-place finishes during his four seasons in charge.
Tottenham have failed to match those heights since, with Andre Villas-Boas, Tim Sherwood and current boss Mauricio Pochettino all failing to finish in the top four.
"Tottenham have been a major disappointment since I left," Redknapp wrote on Kicca.
"I had two top-four finishes, we finished fourth twice and should have finished higher in all honesty.
"I thought if we had bought a striker in the last year at Christmas we'd have been very close, I think, to the top
We didn't invest in a striker at that time
It's difficult, that top four is difficult.
"But this year, if you talk Tottenham, it's been a disappointing year for Tottenham.
"I'm a big fan of the manager, I like him very much but if you look at the team this year it certainly doesn't excite you."
Tottenham sold Gareth Bale to Real Madrid for around £85million in 2013 but the players brought into replace the winger have struggled to fulfil expectations.
Under Pochettino, Spurs have instead turned to their home-grown talents as Harry Kane, Nabil Bentaleb and Ryan Mason have all replaced expensive imports.
"They've been carried by three or four of the kids who have done really well," Redknapp added.
"The signings, almost to a man, have been a disaster.
"All that £120m or whatever they spent from Bale and (Luka) Modric has been pretty much wasted really."
Redknapp resigned as QPR manager in February before Rangers finished bottom of the table and were relegated to the Championship.
Source : PA
Source: PA