Watch: Steyn red-carded for kicking opponent
The Springbok was sent off in the first half of the log-leaders' 12-9 defeat at Lyon on Friday.
Steyn, who has played 59 Tests for the Springboks, had confidently kicked Stade into a 6-0 lead courtesy of a 40m penalty and a quick-thinking drop goal.
But after Mosese Ratuvou had cut the deficit with the night's opening try, Steyn saw red in the 31st minute for kicking the Fijian wing as the two tussled on the ground.
It left Stade to play 50 minutes with 14 men and Lyon took full advantage when Ratuvou added a second try early in the second period, muscling his way over after a sharp pass from former Springbok scrumhalf Ricky Januarie.
Jerome Porical kicked the extras for a 12-6 lead.
Stade's
replacement flyhalf Jules Plisson kept his side in
the hunt with a well-struck, long-distance penalty on the hour mark. Plisson had a chance to level the match with 10 minutes left but pulled his penalty wide of the posts.
Steyn said he didn't understand why he got a red card. He told French newspaper Midi Olympique that it was at best a yellow card offence.
"The Lyon player tried to put his hand in my eyes. I was beside myself. I hate that kind of behaviour. Those are things you shouldn't do, I was angry and reacted badly. But was it worth a red card? Honestly, it's hard," said Steyn.