The Bulgarian international has struggled to make a consistent impact ever since his £6million move to Villa from Celtic 18 months ago.
The Irishman said: "I've decided to take Stiliyan out of the firing line at this minute to get his confidence back.
"To be fair, we've played him out of position because he can go and play off the centre forward. At the moment, he's not involved, but he's got a great determination about him. I know him very well and he's very strong mentally.
"He knows himself that he's capable of so much more when his confidence is high. His confidence is a bit low at the moment but he'll bounce back. I've got no doubts about it.
"I've seen him in games where he might lay the ball off whereas before, when full of confidence, he would thrust forward with the ball. Those things will come back.
"How long will he be out? He's part of the squad and people are fighting for positions and, even though our squad isn't that big, that is great news."