Fernando Santos played down Cristiano Ronaldo's gesture after Portugal missed out on automatic World Cup qualification, though the under-fire head coach guaranteed his team will be in Qatar next year.

Portugal will feature in the World Cup play-offs following Sunday's dramatic 2-1 defeat at home to Serbia in Lisbon.

Requiring only a draw to book their trip to Qatar in Group A, Portugal were on track after Renato Sanches and Dusan Tadic traded goals in the opening half.

However, Aleksandar Mitrovic came off the bench and scored in the 90th minute to send Serbia to the showpiece tournament.

Portugal superstar and captain Ronaldo cut a frustrated figure afterwards, gesturing as coach Santos approached him on the pitch post-game.

But Santos insisted Ronaldo was referencing March's reverse fixture in Belgrade, where the five-time Ballon d'Or winner was not awarded a stoppage-time goal, despite the ball appearing to cross the line in the 2-2 draw.

"Nobody was explaining anything," Santos told reporters when asked about Ronaldo. "He was telling the other guy that there [in Serbia] he had scored a goal at the last minute and that the referee didn't count."

"That's what he was saying at the time. It wasn't a matter for us to be there now. On the field, he said that we scored a goal there and the referee didn't, but it's his outburst. It's perfectly normal."

"Now no one goes after a game to explain what happened or didn't happen. I went to raise the players' heads."

Ronaldo's Portugal will take part in the play-offs as one of the second-placed seeded teams.

The play-offs will not be contested in two-legged ties, with the semi-finals and final set to take place in March next year.

"The truth is that we did not have a good performance overall," Santos said. "Therefore, we have to say sorry. The Portuguese people are obviously sad. Not more than us, but just like us."

"But my team will be at the Qatar World Cup. That is guaranteed. We just have to compete in the play-offs which is not common for us. Portugal never went through a play-off with me, but they played it on three occasions and qualified for the finals in all of them."

"We will do it again. And I personally took part in a play-off, too. And I also qualified for the World Cup."