The Magpies drew 0-0 with last season's Portuguese top-flight champions.
"We know they're a big European club who have the quality and the they love to play football so we knew the game would be very difficult," said Atsu. "I think we defended really well in the first half and we were tactically disciplined. They had more ball possession than us but we needed to stick to the way we wanted to play, to defend and hit them on the counter-attack.
"It was a presentation game for them (the whole squad was introduced to the supporters before kick-off) so they had to show their fans they were ready to go in the season. They are playing in the Super Cup (final) at the weekend, so everybody had to make sure they were ready for that.
"We are going to meet these kind of teams in the Premier League when we play Man City and Arsenal who(tend to) have more ball possession so we need to stay compact and try to create chances ourselves."