The former Paris Saint-Germain star arrived at Old Trafford in a free transfer on deadline day.
Many initially questioned the decision to sanction a move for the 33-year-old, with Ole Gunnar Solskjaer searching for more attacking reinforcements.
But he announced himself to the Premier League with a stunning second-half cameo that score him register one assist and two goals, including a stoppage time winner at St Mary’s.
Cavani came off the bench with United two goals down following strikes from Jan Bednarek and James Ward-Prowse.
However, after Bruno Fernandes got United back in the game, Cavani took centre stage, scoring two well-taken headers.
Following the game, United legend Keane was full of praise for the Uruguayan, suggesting it could be a transfer gamble that pays off in a big way for Solskjaer.
Asked if Cavani has what United have been missing, Keane told Sky Sports: “I think so. I think it was always a gamble when he came to the club.
“People were doubting why he didn’t have a club, did he have the hunger and desire?
“We’ve seen there clips of him in some other games and you think, ‘Has he still got it?’
“We’ve seen proof of it today and he could turn out to be a huge signing for United.”
As has become customary, Keane did aim a small jibe at United’s saviour after he appeared to have the wrong boots on before coming on.
How many goals will Edinson Cavani score in the Premier League this season? Have your say here
Cavani was seen swapping studs and still tying his laces when he ran onto the pitch.
“He was obviously slow getting on the pitch with them boots, just as well he did score because…how the guy is not prepared and ready to go on – it’s mind boggling,” Keane added.
“Obviously he didn’t need the boots in the end scoring two headers, great contribution when he came on. He still has that quality and that hunger and desire.”
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