The Whites lost 2-0 and have now been beaten in four of their last five league games and failed to score for the fifth time in their last six matches.
Their one-time 11-point gap over another of the promotion chasers, Fulham, is now down to none and Leeds could well drop out of the top two this week with further tricky games away to Brentford on Tuesday and at home to Bristol City on Saturday.
United's disastrous run has seen them concede first in every match they have played in 2020 and while their failings in front of goal have been well documented they are now leaking at the back as they have failed to keep a clean sheet in 10 league games - In the 10 matches before that they kept a blank seven times and only let in three.
But despite the obvious downturn head coach Bielsa once again said it was down to him to find solutions.
He said: "The team made a big effort. How the team ran, all the effort the players did, really touched me.
"We had better players than the opponent, but what we did wasn't enough to avoid another defeat. Trying to give an explanation now is not useful.
"The resources the team have is enough. We cannot more demand more effort from the players. So clearly it is not player responsibility at this moment, it is necessary to say that the responsibility for this moment of the team is mine because clearly if you have good players and those players give all on the pitch, the conclusions are clear. I have to understand that it is me who has to find the solutions.
"We can't demand any more effort from the players.
"I'm prepared to find solutions to turn this negative situation into a more positive one.
"It was a very close match where none of either team had a lot of possibilities. What both teams did in attack was quite similar. But of course we needed goals and they scored once before us. If we didn't create the same chances we used to create that makes the situation worse. It is for this reason I feel responsible."
Forest went ahead thanks to a series of defensive errors, including one by under fire goalkeeper Kiko Casilla who let Sammy Ameobi's shot beat him at his near post - but Bielsa would not lay blame at his keeper's feet.
He added: "When you analyse football you cannot analyse football without mistakes.
"If it was a mistake, maybe this provoked the goal, but this mistake doesn't provoke the defeat.
"Of course I analyse and I am forced to take decisions but also when you take a decision it has to change in a positive way the reality you want to change. It is necessary to find the problems and not make wrong decisions in the solution you propose. In general, there are solutions the teams need and I'm not getting that yet.
"All what I answer around Casilla in a moment where Casilla has offered answers that weren't enough, it would be unfair if I analysed those kinds of actions. Keepers of course they make mistakes and big top goalkeepers are not top goalkeepers because they don't make mistakes, but for the capacity to correct themselves in bad moments. We have different ways to manage the situation, I think that I cannot condemn in public the situation of Casilla."
Leeds had plenty of possession as usual, but did not use as well as usual as their passing was rushed or sloppy and they took too long to get their football together in the first half.
Forest put in the early efforts, but did not fashion a real chance although Casilla was forced to make saves from shots from distance by Tiago Silva.
United looked to be finding their feet only for Patrick Bamford to put in a woeful header after meeting Pablo Hernandez's cross and Ezgjan Alioski to see his shot deflected wide.
They then fell behind as Hernandez lost the ball and could not win it back as Ameobi strode into the box to beat Casilla with a powerful strike, but one that would have been saved had the Whites keeper got his positioning right.
The response was not good with only one further effort from the visitors as Hernandez's 20-yard shot took a deflection and was easily saved by Forest keeper Brice Samba.
They did improve eventually after the break, but not instantly as Lewis Grabban should have put the hosts two up after the ball fell to him in the box following a good run by Joe Lolley and he shot straight at Casilla with the goal gaping and the United keeper already on the ground having committed himself too early.
Leeds' night was summed up soon after when Jack Harrison's possibly goalbound volley hit teammate Bamford, who was offside and a chance had gone begging.
They did enjoy their best spell of the game as it went into the final third and should have equalised when Liam Cooper had a free header from a corner, but he could only direct the ball at Samba. Forest's night was summed up as the stopper could only parry the ball and was lucky to just prevent the ball from crossing the line with a second save.
New signing Jean-Kevin Augustin came off the bench to make his debut with Bamford the man to make way for him and he looked a more direct presence, but only managed one effort, which was easily blocked.
Helder Costa was judged to have headed wide at the far post, when it looked to come off a home defender, while Mateusz Klich was inches away with a low shot from a tight angle.
The big finish did not really materialise, though, and it was Forest who scored a second goal completely against the run of play deep into added time as Harrison failed to deal with a big boot out of defence by the hosts. Lolley took the ball off him and ran clear before unselfishly setting Tyler Walker up for a simple tap-in.
Leeds' misery was subsequently complete and their alarming run now makes promotion look a long way off.
After wasting another transfer window and apparently failing to improve the team there are some now doubting whether United will match last season in reaching the play-offs.
Nottingham Forest 2
(Ameobi 31, Walker 90+5)
Leeds United 0
Forest: Samba, Cash, Figueiredo, Worrall, Ribeiro, Watson, Ameobi (Diakhaby 67), Sow, Silva (Yates 80), Lolley, Grabban (Walker 90).
Leeds: Casilla, Ayling, White, Cooper, Alioski (Shackleton 70), Dallas, Costa, Klich, Hernandez (Roberts 59), Harrison, Bamford (Augustin 70).
Referee: Oliver Langford