The Macedonia international was sent off three minutes into the second half in the match at The Hawthorns for spitting towards Kyle Walker.
Popov apologised to fans via Twitter after the game, but has now made a full apology.
The 28-year-old said in a statement released by the club: "I just want to say a big sorry to everyone, firstly to the gaffer because he was so angry after the game and, if I was in his place, I would be exactly the same, and to all the players."
"We played a good first half then at the start of the second half their player kicked the ball to the corner and I said something to him.
"We exchanged a couple of words but it wasn't something bad, just something that happens 10 to 15 times in a game.
"After that, when he started to run, he was maybe two or three yards away from me. I didn't have the intention to spit on him - but it looked like that.
"The referee was close to me and saw that. This is not good for me because over the last two or three nights I have not slept too good.
"Something like this has never happened in my life."