Popov was shown a straight red card by referee Mark Clattenburg after the incident early in the second half.
Baggies outcast Peter Odemwingie, left out after trying to force a move to QPR on deadline day by driving to Loftus Road, was on Twitter a couple of hours after the game to reveal Popov's apology.
He said: "Just had Popov the Albion man on phone. He is not on twitter but asked me to pass a msg to Albion fans that he is sorry for the red card.
"Goran Popov: sorry for letting the team and the fans down. All players worked very hard and his mistake cost the team 3 points.
"What happened to Goran happens once in years. He was the player of the month last month. Thats life. Ups and downs."