Bryan Mbeumo (2) and Yoane Wissa struck for the Bees as Ivan Toney watched from the stands after being sold to Al Ahli yesterday.
Frank later said: "They made errors because we pressed them unbelievably well. I think we are one of the best teams to do the high pressure in the Premier League, in my opinion. the players are so committed and that is the reason why we capitalised on errors from Southampton. It's up to us to take the chances. I still think we missed some big chances.
"Overall, I'm satisfied, always want more but we are on track with how we want to play and how we want to do.
"Ivan is a fantastic player, I'd love to have him in the team but he has his own journey and going elsewhere. Our style of play hasn't changed. Wissa and Ivan are different players but the style doesn't change.
"It seems like I don't want to talk about Ivan (Toney). I love to talk about Ivan but it seems like it's about when he's going to leave or a ban. He's a wonderful player and person."