Newcastle boss Rafa Benitez watched his side go down 1-0 at Fulham and then described it as a "wake-up call".
The Magpies were beaten by Matt Smith's first-half header in a lacklustre Championship season opener following summer spending of almost £45 million.
Benitez knows Newcastle cannot afford many similar performances if they are to immediately return to the Premier League.
"I'm really disappointed because we did not play at the level we expected," he said. "We did not win the ball too much in the second ball and were not keeping the ball properly. We were not concentrating. This game could be a wake-up call for everyone.
"The only positive is it is the beginning of the season and we can wake up. At least everyone has to realise it has to be like this for the rest of the season.
"We didn't play at the level we want to play and have to play if you want to score goals and win games. You have to be much better. We have to realise this is a tough competition and we have to be much better."
Newcastle had two penalty appeals waved away for handballs by Ryan Tunnicliffe and Tomas Kalas but Benitez refused to criticise referee Simon Hooper.
"The first one was clear and not given," he said. "The second one depends on the referee."
Opposite number Slavisa Jokanovic was delighted with three points and a clean sheet having had the third worst defensive record in the Championship last term.
"It's great for us, the perfect start," he said. "I took great pleasure from my team. They played altogether. It's good news, my team played a serious game. If you follow this way we can think about a better season.
"I thought my team worked very well in pre-season. They showed what they had to do.
"We had many difficulties keeping a clean sheet last season, it's great news for us.
"My team is very sharp. But this Championship is so long with so many games. I said before we were ready for Newcastle but not for the season.
Jokanovic revealed that he will be handed the proceeds of Ross McCormack's recent £12 million sale to Aston Villa to bring in new players.
"We need to make investments and it is £12 million more," he said. "I want to spend all of it."
Source: PA
