The Spaniard was asked about his decision to substitute Amadou Onana in a win over Bologna on Tuesday.
As Villa tried to make it three wins from three in the Champions League group stages, Onana made way for Ross Barkley at half-time.
On the sub, Emery said: "Tactical decision but as well he had some pain."
He added: “Not clear. I decided tactically and also not take a risk with him."
"We are playing seriously, we are playing focused, responsible and the team is progressing and mature in everything," Emery said about his team.
"We tried to break the press man to man. We conceded some things in the first 15 minutes but then reacted very well.
"It is always difficult at home in Europe. We have had to try and be consistent in our gameplan for 90 minutes and I think we did that today."