Redknapp has labeled the deal as a gamble and insists the English side were desperate.
Manchester United were reportedly unable to land their primary targets during the summer transfer window but enjoyed a busy deadline-day following the 6-1 defeat to Spurs 24 hours earlier.
The Red Devils signed Cavani, who left Paris Saint Germain following the conclusion of his contract in the summer.
The Uruguay international is said to be on a sizeable weekly wage, a decision which was castigated by the English coach.
“I think you all know I like a gamble now and again...but nothing like the one Manchester United have taken in signing Edinson Cavani," Redknapp wrote in a column for The Sun.
"It was obvious they needed something even before last week’s shocker against Spurs. The worrying thing was a few of them seemed to throw in the towel.
"But signing Cavani, 33, on over £200,000 a week on the final day of the transfer window did smack of desperation to me.
"I had people telling me eight weeks ago he was keen to come to England. When I heard the wages I just wished them good luck.
"So if he was someone United were that keen on signing, why did they leave it so late?
"Cavani has been a great player over the years and if he’s still got the hunger and the talent, then he’ll do a good job for them.”
“I hope for Ole Gunnar Solskjaer’s sake he is because they certainly cannot afford any more displays like that one against Spurs."
