Long opened the scoring for the Saints before Mane scored a hat-trick to complete a stunning victory for the Saints.
Tadic was a constant nuisance for City's defence as he provided three assists in the game and the threesome received high praise from their boss.
"We have a lot of competition in our forwards and I think the three players up front did an amazing job today," Koeman told Sky Sports.
"Dusan is a type of player where if he gets some freedom, he's dangerous. He sees the options to give at the right moments, that's one of his big qualities."
Need a VPN? Or seeking to switch? Then support Tribal Football by clicking here to sign up with our official partners NordVPN. Not only will you receive the best VPN experience, but also great discounts by signing up via Tribal Football. Thanks for your support!