Guardiola was visibly frustrated with his side for throwing away their three-goal lead.
The former Barcelona manager had several scratch marks on his forehead during his post-match interview with broadcaster Amazon Prime.
Asked by reporters during his news conference about a mark on his nose, Guardiola said: “I cut it with my finger, my nail.”
Guardiola then said that he wanted to “hurt himself” because of another disappointing result, before laughing and leaving the news conference.
City have not kept a clean sheet since 26 October, when they last tasted victory in a narrow 1-0 win over struggling Southampton.
Since then they have leaked 17 goals in six matches, an astonishing statistic considering their success under Guardiola has been built on solid defensive foundations.
“I don’t know if it is a mental thing,” said Guardiola.
“The game was fine at 3-0, playing good, but then we concede a lot of goals because we were not stable. We gave them the first and then the other one, that is why it was difficult.
“The situation is what it is. We played a good game but at that level we can’t give them away.”
City have work to do if they are to avoid an unwanted two-legged play-off in February to progress to the last 16.
They have to finish in the top eight to earn an automatic passage into the knockout stage but sit 15th after Tuesday’s game. They could fall even lower after Wednesday’s matches.
City face two testing games at Juventus on 11 December and Paris St-Germain on 22 January next in the competition before hosting Club Brugge on 29 January.
“Right now I am not ready to think about that [needing to win their final three games to finish top eight],” added Guardiola.
“We have to recover and prepare for the next game. If we are not able to win games like we did today it is difficult to do it.”
