Inter Milan boss Andrea Stramaccioni conceded the 3-1 home defeat by Roma could prove psychologically damaging to his side.
The Nerazzurri opened the Serie A season with an encouraging victory at Pescara last Sunday as they look to claim the Scudetto for the first time since 2010.
But they found Roma a different prospect with Zdenek Zeman's men turning in a performance which was a significant upgrade on their opening-weekend 2-2 draw at home to Catania.
Alessandro Florenzi gave Roma the lead in the 15th minute before a deflected shot from Antonio Cassano brought Inter level in first-half stoppage time.
Daniel Osvaldo restored Roma's lead midway through the second half and Marquinho added a brilliant third 10 minutes from time.
Stramaccioni said: "I didn't like the decline we suffered. In my opinion we had the game in our hands. We had kept our shape and were compact. We had also come back onto the pitch with the right attitude.
"Then there was that strike of lightning, that deadly counterattack that gave them their second goal, and after their second goal things were no longer going well for us.
"It came down more to something psychological than something in our game.
"It's a setback but only a psychological one."
Veteran full-back Javier Zanetti believes teething problems are inevitable with such a new-look side.
He said: "We knew that our opponents would be good at taking advantage of the space we would leave from time to time. It's just that tonight we did it too frequently.
"We all need to focus on understanding what's going on. We're just at the beginning. We still have to grow a lot and there's much to correct.
"We have many new players and we have to understand just what the coach wants and how to put it into practise in order to start winning again.
"Roma have proven to be a very solid team and I think they can go far this season, and I hope we're there too."
Zeman was pleased with his side's display and reserved special praise for experienced playmaker Francesco Totti.
He told www.asroma.it: "We played a good game, and for stretches we played very well. The result should give us the belief that we can compete against anyone.
"Everything we asked Totti to do on the pitch he did well. For me he's a great player.
"He started as an attacker on the left, then gave his contribution in midfield, in defence..."
