Three goals in seven second-half minutes turned a frantic match Forest’s way as Bielsa suffered back to back losses for the first time as United’s head coach.
The Argentinian had earlier conjured another dramatic fightback as Leeds overcame an early concession and a 42nd-minute red card shown to Kalvin Phillips to claim a 2-1 advantage midway through the second half.
United, however, have conceded 10 times during four matches over the Christmas period, a marked increase for a side who shipped just 18 in their first 22 matches.
Leeds were punished twice from corners by Forest and Bielsa, who has been using Phillips as a centre-back while Liam Cooper and Gaetano Berardi recover from injuries, admitted defending at set-pieces was a weakness in his team.
The January transfer window is open but Bielsa, whose squad’s lead at the top of the Championship was trimmed to two points, has previously dismissed the possibility of adding another defender to his ranks and maintained that stance after the result at The City Ground.
“It’s not a new fact that we have problems on set-pieces and it’s not a new fact that we make it easier sometimes for the opponent to have chances to score,” Bielsa said. “But the game has these kind of risks and the fact that we dare to play, sometimes it has negative aspects. It also gives us a style of play that we should be faithful to.
“We don’t have any problems in defence. If you take out the goals that are scored at set-pieces, we are the team who have conceded the least. Obviously set pieces are part of defending and we can’t hide the fact we’re lacking good headers like Berardi and Cooper.
“We made some corrections and the solutions we found are not final solutions but I don’t think we can solve this problem with a new player.”
Bielsa admitted he had considered introducing an additional defender in Jamie Shackleton after Gjanni Alioski volleyed United 2-1 ahead on 64 minutes but decided against the substitution, saying it would have forced him to remove an attacking player.
“When we scored the second goal I thought about making a change to put a defender on but honestly, I didn’t take a decision because I didn’t find any player who could be useful at that moment,” he said.
“If I had taken this decision I would have thought that I was destroying something which was working well.”
Phillips is facing a three-match ban after incurring a straight red card for a foul on Forest’s Adlene Guedioura three minutes before half-time.
A studs-up tackle caught Guedioura late but Bielsa said: “I don’t think the intention was to hurt the opponent and it’s hard for me to condemn this decision (Phillips’ challenge).
“With this kind of decision, even if it’s not prudent, the ambition was to reach the ball.”
Bielsa also refused to condemn referee Darren England’s failure to send off Forest defender Jack Robinson in the 14th-minute after he bundled over Jack Harrison as the winger broke clean through.
Leeds were trailing to a goal from Jack Colback but were dominating possession and frustrated by the yellow card shown to Robinson.
“Any opinion I make on the decision of the referee, especially after a defeat, would look like an excuse,” Bielsa said. “We have to impose our style no matter the behaviour of the referees.”