Graham Potter was disappointed with his side's second-half performance but said that Albion have to accept the VAR's decision to award Southampton what proved to be a winning penalty.
With the game poised at 1-1, David Coote awarded the visitors a free-kick on the edge of the box before the VAR Peter Bankes ruled that Solly March's tackle on Kyle Walker-Peters took place inside the box.
Danny Ings converted from the spot to ensure the Saints took all three points with a 2-1 win.
“I haven't seen it back,” Potter said. “It looks like the first challenge is outside the box and then there is a second challenge, that's what the referee seemed to think and if that's the case then that's how it is.
“If it's factually inside the box and the offence is deemed to be inside then that's what we're using VAR for I suppose. It's just one of those things you have to accept.”
Albion had led through Pascal Gross' penalty before Jannik Vestergaard levelled just before the break.
“We're a little bit disappointed with our second half performance. It was a pretty even game and there wasn't too much in it. We had some opportunities, but I am disappointed with the result.
“It was an even game. We started really well in the first 30 minutes and then they came into it in the last 15, and it was quite even in the second half.
“It's hard in a Premier League game to control all aspects for 90 minutes. You know the opponents are going to have some time and they did. We weren't able to survive the first half and that little bit of pressure which is disappointing.”