Frank also admits he knows there will be a fans' backlash after the 4-1 hammering.
He reflected afterwards: "I think it is extremely painful. I won't talk away from that. It was a bad performance. It was completely the opposite of what the intention was when we came here. We can only apologise to the fans for the performance. I think no matter if both teams wanted to play shirt, they got more out of that and we couldn't get out.
"When the team went long, we didn't win enough duels. That is exemplified by the 2-0 goal and the 3-0 goal, where a player went through two or three players. Bad performance and we lost.
"I have seen a lot of character and fight in this team but we didn't win enough duels. We can call that whatever we want but we didn't win enough.
"I tried to play a 5-4-1. I will always take responsibility as my choice in the end to take the system then and we changed it at half-time. My vie won it is that no matter what formation we play today, we don't have enough duels or intensity in the decisive moments, and it is very difficult to win a football match.
"The goals I want to see more in detail. The whole game I need to watch tonight an it will be a difficult watch. We were not aggressive and stepped forward when we could and they played it round us.
On lack of creativity from his players, Frank also stated: "That has been an ongoing theme that we are working hard to improve. It doesn't look good today or against Chelsea. We need to keep working on it.
"There were a lot of things in this game we need to do better. We are four months into it and they are further in their journey as a team and that was very obvious today.
"Of course there will be noise. We played against our biggest rivals and we lost badly. But we keep noise out and we focus. I know this tam is very competitive. I know this team is competitive and we showed that against Man City and PSG. Of course it looks bad today and it was not good enough."