The Brazilian made amends for missing an earlier easy opportunity when he arrived on the end of substitute Alexander Isak's perfect low cross to make the decisive contribution with 17 minutes left.
Newcastle will now be favourites to reach their first major Wembley final since the FA Cup in 1999 as they attempt to end a trophy drought stretching back to the 1969 Inter-Cities Fairs Cup win.
Toon midfielder Bruno Guimaraes later said: "We controlled the game but we missed big chances.
"It's just the first game now we need to think about the next one. I'm proud of the team today.
"One goal is important, we could score more but most important was to be one-goal in front of Southampton.
"I want to thank Nick Pope I think he's the best goalkeeper in the world.
"Let's keep it going."