Keane has claimed that the South American could be "a huge signing for United", who have now been victorious in their last four matches in all competitions.
"He was obviously slow getting onto the pitch with those boots," Keane told Sky Sports News. "Just as well he did score because how the guy isn't prepared and ready to go on is mind boggling.
"Obviously he didn't need the boots in the end, he scored with two headers and had a great contribution when he came on. He obviously still has that quality, and the hunger and desire.
"It was always a gamble when he came to the club. People were doubting why he came to the club, did he have the hunger and desire? We've seen clips of him in some other games and we thought, 'Has he still got it?', but we've seen proof of it today, he could turn out to be a huge signing for United. It's worked out well today."
Cavani has now scored three times in five Premier League appearances for the Red Devils. body check tags ::