He said: "I just told him to imagine how many players would like to be in his position. He's not been the way that I know Shaun from Celtic. But he just needs to cope with that, get his head up and start putting in performances."
The 24-year-old made just six league appearances for Villa last season, scoring against Manchester City in the last of those.
"That was the first time I'd seen a smile on Shaun's face," said Petrov. "He was so happy afterwards at scoring a great free-kick and he put in some excellent performances towards the end of the season."