Ferguson opted to select a youthful defence in which Chris Smalling was the oldest component at 22. The move nearly backfired spectacularly for, despite taking an early lead through Javier Hernandez, United conceded either side of the interval and by the end were one goal away from going out.
"I have to accept responsibility," said Ferguson. "Playing so many young players in the back four was a big ask."
He added: "I thought with a 2-0 lead it was a great opportunity to give Chris Smalling and Phil Jones a game because they have been out for such a long time.
"They have great potential but the lack of experience in terms of tying up the game created a problem for us.
"For a European night it was too much of a risk. We survived it with a bit of luck but we didn't play well in the second half and it was a nervy night."
United only held on thanks to a superb David De Gea save from Siem de Jong at a time when Ajax were dominating.
It was yet another example of the young Spain star's superb reflexes, which have got his side out of trouble on more than one occasion this season.
"De Gea made a tremendous save," said Ferguson.
"You think about the size of our team and they got two headers from corner kicks, which you don't expect."
Source: PA
Source: PA