Middlesbrough boss Tony Pulis was left frustrated after Leeds United snatched a point deep into added-on time in a dramatic Championship clash.
Kalvin Phillips scored late on to seal a point for Leeds, the game finishing 1-1 at the Riverside Stadium.
Phillips was on hand in the six-yard box to head over the line in the 11th minute of added time after an incident in the visitors' dugout in the latter stages of the game on Teesside.
Leeds' teenage winger Jack Clarke, who had been withdrawn at half-time, had to be taken to hospital after feeling unwell as he watched the second half.
The game was stopped for seven minutes and led to referee Darren England asking for 12 extra minutes beyond the 90 when Middlesbrough led through Lewis Wing's opener soon after half-time.
And it was at the end of stoppage-time when Leeds midfielder Phillips was in the right place to head his side level - and lift Bielsa's team back to the top of the Championship.
Pulis was left frustrated by the manner of the result, even though he was also pleased to see his side recover from the midweek disappointment of losing at Newport in the FA Cup.
Had Middlesbrough held on then they would have climbed to within four points of the top two and beaten both Leeds and West Brom in the space of seven days, but they at least head to Sheffield United on Wednesday with four points from those two matches.
Pulis said: "It was disappointing because it is late, and when it comes late in the game it is always hard to take. I wish my players had seen it coming.
"I thought the players were fantastic first half. Our midfield players were top drawer - Mikel was the best player on the pitch by a mile for me. Saville, Wing and Howson really, really got round Jordan Hugill and caused them all sorts of problems.
"Coming in at 0-0 at half-time I was disappointed because I thought we should have been in front. Second half I thought Leeds were the better team, I thought at times they really pressed us and the lads dug in having got the goal.
"I am more concerned about Jack Clarke and I hope he is OK. The time that was added on, was the time that the referee decided and it is the same for both teams. It is set for Leeds as much as it is set for us.
"The more important thing is making sure the boy, fingers crossed, is OK and recovers because he is a very, very talented young player. All of our thoughts for Middlesbrough Football Club go to the lad Clarke."
Marcelo Bielsa felt that Leeds created enough chances to win.
"What we know is that he didn't feel well," Bielsa said. "We don't really know the reasons yet.
"After feeling bad he recovered and when he was taken to the dressing room he was better, as if he had overcome the problem. He was taken to the hospital to see if everything was alright."
Bielsa added: "The opponent was dangerous in the first half but at the end of the first half and the second half was good. We could have won the game.
"If we take into account the 15 minutes in the first half when we were not at ease, apart from this we dominated the game. We deserved to score that goal and we probably deserved to score more goals."
Clarke was taken to hospital after complaining of illness in the second half.
There was a lengthy delay after the 18-year-old, who was substituted at half-time, was taken ill in the dugout and then stretchered off while given oxygen.
Bielsa revealed after the game that the promising teenager had shown signs of improvement before being taken to hospital.
Leeds later added in an official statement: "Jack Clarke began to feel unwell during the second half of our Sky Bet Championship fixture with Middlesbrough and received medical attention.
"The player is responsive and attending hospital with the club's head of medicine and performance.
"On behalf of everyone at Leeds United, we would like to thank the medical staff from both clubs for their response to the incident."