Mason showed Sadio Mane a straight red card in stoppage time after catching Erik Pieters when jumping for a header. Koeman expects Southampton to appeal Mane's dismissal.
Koeman said: "In my opinion, it isn’t a red card. If everyone in the club agrees with the manager, then we will appeal against that."
The Saints boss also said that Mason and his assistant referee missed a clear penalty with Dusan Tadic appeared to be tripped by Jack Butland in the penalty box in the second half.
"I had the chance to see the clip, it’s an easy answer. It’s 100% a penalty. It’s difficult decision, I understand, but the referee and the linesman need to see what happened."
Koeman will be glad that two seemingly poor decisions didn't cost his side the three points in what was a huge result against Stoke City at the Britannia.