Rashford was again impressive for Villa in last night's 2-1 defeat at Manchester City, converting a penalty for the visitors.
The England international joined Villa on-loan in January to the end of the season with a permanent option worth £40m.
Keane told Sky Sports afterwards: "He was a threat tonight, he had a really big chance in the second half but when there is grass in front of him he is as good as any player.
"Ask the defenders, he is a nightmare. It is almost impossible to stop."
Costly defeat for Villa
Keane also said: "The problem for Villa is they are running out of games and the goal difference is a problem. They are well organsied, but they give up late goals and that will be costly.
"There will be one or two twists but where you are in a position to draw, don’t lose, keep the momentum and City near you.
"You have seen the reaction of city, they know what a big moment it was last kick of the game."