The incident came in the 76th minute with Rovers looking comfortable and heading for a point from the title chasers.
Nzonzi was then shown a straight red card for a two-footed challenge on Laurent Koscielny, the Rovers boss admitted after the game it was a decision which could have gone either way.
"When I first saw it looked a bit nasty, but when I've seen it again got some of the ball," Keane said.
"But it is one of those which is sometimes a yellow and sometimes a red.
"The player is devastated because he is not a malicious player and thought he made a genuine attempt for a 50/50 ball."