Mowatt was an unused substitute for Leeds when they were beaten in front of the Sky cameras, courtesy of a Conor Hourihane free-kick, then he joined the Reds the following week.
He said: “The atmosphere was unbelievable. I remember the Hourihane free-kick. The atmosphere when Leeds scored was crazy then when Hourihane scored it was crazy in the Barnsley end.
“That was one of my last games at Leeds. It was a good game, Barnsley won and hopefully they do it again on Sunday.
“It was strange because I knew Barnsley were interested in me. I was watching them to see how they played. They were class that game and all season. Their performance in that game and the atmosphere made me want to join them.
“In that game Barnsley were the underdogs and I think we will be this weekend as well. We have nothing to lose and we can go out and enjoy it.”
Sunday’s match is the first time Mowatt will face Leeds, for whom he played 125 games after coming through the academy.
“I am looking forward to it. It will be a good game in a derby atmosphere. There is definitely a different atmosphere around the club and everyone is excited.
“You always want to win every game but when it’s your former team you’re dying to play well and win.We just have to make it a derby game. We didn’t start well in our last derby game (a 2-0 loss at Sheffield Wednesday). We have to make it aggressive and high tempo.”
As well as Mowatt, Barnsley have former Leeds players Aapo Halme, Clarke Oduor and Mallik Wilks. Head coach Daniel Stendel said: “Our players who came from Leeds are very motivated to show them and the manager that they are good players. We have four players from Leeds but we need 11 players who are motivated in the game.”