Grealish was knocked to the ground after being struck by a supporter who raced onto the field in the 10th minute at St Andrew’s.
But the boyhood Villa fan responded by firing home the only goal of the game in the second half.
Hourihane said: "It was unbelievable from Jack. He did incredibly to keep calm.
“For an incident for that to happen and to keep his cool. At the end of the day he has had the last laugh and I'm delighted for him."
Hourihane branded the incident as “disgusting” and added: “Stuff like that cannot be tolerated. Jack could have been very, very badly hurt.
"When he ran on the pitch I saw him from the corner flag and he was running towards us.
“I thought here we go, it's a streaker, albeit with his clothes on. No-one expected what happened to happen.
"It's not easy to keep your mind on the game. It is one of those things, a freak incident. You have to try and get on with it as best you can.
"We had to refocus pretty quickly. Obviously the incident is in the back of your head for a few minutes but the game goes on. We acquitted ourselves really well."