McAuley welcomed back Popov and he revealed his return has shocked everyone.
"We didn't know until he just turned up and surprised us, and that's him," McAuley told the Birmingham Mail.
"It's great to have him back. He's lively around the dressing room and he was an important player last season.
"It's a change coming from Ukraine to play in the Premier League. Now he's in his second season and the experience of last year will be invaluable to him.
"The red card was unfortunate and he's learned from that.
"He realised he lost his place in the team when he was getting in there and when you do that at a club like this, there is always somebody ready to take your slot."