Mounie hit the post during the 3-0 reverse.
"We had a very good first half, but we know that Chelsea, when you face a team like this, they are very clinical," Mounie said.
"You have to use your chances and (luck) wasn't on our side unfortunately, so we keep going and keep working, it's only the first game."
Mounie took exception to Jorginho's cheeky penalty, adding: "It was weird.
"He jumped before the penalty and Ben (Hamer) had already dived so I don't know why the ref didn't make him retake the penalty.
"It was a little bit weird. The ref needs to explain that to us because I don't understand why (it was not retaken)."