Thomas Tuchel has taken responsibility for asking centre-back Andreas Christensen to deal with Vinicius Junior in Chelsea's 3-1 defeat by Real Madrid.
The Brazilian ran the Danish defender ragged, exploiting the space between him and Blues wing-back Reece James in the first half at Stamford Bridge to devastating effect by setting up the first goal of Karim Benzema's stunning hat-trick.
The 25-year-old Denmark international's torrid evening was ended when he was replaced at half-time by midfielder Mateo Kovacic, while England star James was instructed to play deeper in the second half to negate the threat posed by Vinicius.
But damage had already been done with Benzema adding a second three minutes after his first header, and the Frenchman - after Kai Havertz pulled one back - completed his hat-trick early in the second half following an Edouard Mendy error.
But asked after the game if it had been an error to allow Vinicius space between Christensen and James in the opening 45 minutes of the Champions League quarter-final first leg, Tuchel said: 'Yeah, it's my mistake.
'We changed the formation and wanted to turn things around because I think personally there was enough where we could hurt them.
'But we were so far off our level in everything what the game demands, tactically, individually, in shape, in stiffness, in challenges that we tried to play in a new formation and then killed the game off with a big mistake after three minutes [in the second half].'
But Tuchel refused to individually call out Christensen and N'Golo Kante - the other player taken off at the break by Tuchel to be replaced by Hakim Ziyech - for their performances.
Asked for the reasoning behind the substitutions, Tuchel - who also said the tie is now over ahead of the second leg in Madrid next week - added: 'No, it was a tactical change.
'It's like always, it's never on one player, it's never on two players, it's on all of us, I'm included in this.
'Like I said, individually we lost shape and sharpness and obviously since the international break we don't look the same.
'I don't really have an explanation because before we had a long winning streak and were very competitive, the first half by far was not good enough.
'If we have the same talk we had five days ago we will still talk about the same defensive performance. But since five days it's been seven goals conceded, I don't know, we did not change anything, not in the approach, the line-ups, the system, so I don't think there is a deeper reason for it.
'It's alarming because it's two games and seven goals, but nothing has changed.'
