The Red Devils’ first Premier League match in three weeks got off to a poor start as Danny Welbeck scored an early opener against his former club.
United were lucky not to be further behind at the break as Brighton eyed a first ever victory at Old Trafford.
Pushed on Harry Maguire’s second-half challenge on Welbeck that was checked by the video assistant referee, Potter said: “I was a little surprised it wasn’t given live because it felt like a penalty and when I watched it back I think it looks like a penalty.
“But once it doesn’t get given, you can understand why VAR don’t overrule it.
“I think if Mike Dean had given it, I don’t think VAR would have overruled it. It is what it is.”