Returning to European action away from home after 53 years, the Black Cats fell behind to Volnei Feltes' spectacular opener.
Jules Ahoka equalised in the 67th minute before Sadiki struck late, sparking celebrations among over 5,000 travelling supporters.
Le Bris described the victory as an important milestone in Sunderland's development.“We are back in Europe after 53 years - and in Portugal again - and winning this game at the end is really special,” he told the club website.
“It is a symbol and really important for many reasons.
“Our fans were fantastic once again today and they are helping us all the time.”
“The team showed this evening a belief and they kept going until the end.
“We showed again this mindset and that is very important for us.”
He added: “As a team and a club we are growing up and we need experiences.
“Our starting XI this evening was just 22, so this team is young but they have huge potential.
“The potential is all about getting more experiences to learn and grow.
“We have to accept it won’t be easy, but we need to show consistency, discipline and work hard every day. The players showed that today.”
Next up for Sunderland is their Premier League outing against Bournemouth on Sunday before heading to Lech in their next Europa League outing.