The 28-year-old Macedonian received a straight red card in the 1-0 defeat against Tottenham, with his manager Steve Clarke labelling the incident as 'disgusting'
Early on in the second half at the Hawthorns, Popov clashed with Walker in West Brom's half and the ensuing incident saw the two exchange words before the Macedonian spat at the Spurs right-back.
Popov's coach Steve Clarke admitted he was disgusted at his defender's behaviour, and the 28-year-old is just as disappointed.
He said: "I am disappointed. 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.
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"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. I've had a couple of good games, was man of the match and Player of the Month in December.
"But I'm not happy with what I did on Sunday. When the referee sent me off it totally changed the game. The players were angry after the game, and some of them – the ones who have been a long time at this club - said to me they are disappointed in me.
"I want to say sorry to all my friends, team-mates, to all the physical staff and the coaching staff. Everyone can make mistakes, life is like this. Football is up and down.
"But Sunday was not good. When I saw it on the video after the game I saw that. Tomorrow I want to make a meeting and I want to say sorry to everyone who works at this club.
"I don't want all the fans, players, trainers and staff to change their mind about me. I'm not that kind of player.
"If I was like that I would not come here and say sorry to everyone. Never in my career have I had a straight red card like this."
The defender currently on loan from Ukrainian side Dynamo Kiev claims he is actively trying to get Walker's phone number, but that has been difficult thus far.
"I'm trying to get a phone number for Kyle Walker but he is now with his national team," he noted. "When I get his number I will call him and just say 'I'm not a player like this, I didn't want to do this to him and a big sorry'.
"From tomorrow I will train hard and try and help my team-mates get a good result. I hope everything will be good because we have a good team, good quality, good players, good staff - like the gaffer - and coaches.
"We train so hard and we train to get something good."