The unfancied Norwegians won 2-1 at Inter to reach the Champions League round of 16 on aggregate 5-2.
Chivu said afterwards: "The Champions League is very competitive, there are teams that if they get to this point they have something.
"They showed it in Dortmund, with Atletico, with City and with us.
"They're taking advantage of the fact that they're not playing and have much more energy than those who play every three days, but that's no excuse.
"We could have done better in Norway, and tonight, it didn't go the way we wanted, but that's football.
"We have to accept it, they beat us twice, and we have to give them credit because they were better.
"My players did their best, but we weren't able to break the deadlock and apply mental pressure."