The 18-year-old had to be tended to by paramedics and was taken to hospital after complaining of feeling unwell late in the second half.
He was discharged late last night, but will have to undergo more tests in the next few days.
Clarke spent the night under the supervision of Leeds' medical staff after the worrying incident.
The attacking midfielder posted on Twitter today after being inundated with get-well-soon messages.
He wrote: "Would like to thank everyone for the support and love they have shown, massively appreciated."
The 18-year-old was replaced at half-time of the 1-1 Yorkshire derby draw at the Riverside that saw Leeds move to the top of the Championship.
The club's chief executive Angus Kinnear has also thanked the footballing world and wider for their support for the teenager.
He wrote on the club's website: "Jack has had messages from football fans from every club you can imagine and we all really appreciate it.
“Jack spent the night under the supervision of the medical staff from the club and he will return home today.
“He will undergo some more tests this week and everything is looking well, but he won’t train for the rest of the week.
“In the meantime we would like to thank everyone who has sent well wishes and once again thank Middlesbrough Football Club for all they did for Jack.”
Clarke's agent and former Leeds defender Ian Harte confirmed the player was "happy and smiling" last night after leaving hospital.
Harte posted an update on Twitter, saying: "Jack has left the hospital now happy and smiling. Hopefully back on the pitch very soon."
The Irishman also thanked Middlesbrough medical staff and Leeds' head of medicine and performance Rob Price.
Clarke, who had played the first half, was given an oxygen mask before being taken to the dressing room — where he felt better — and then hospital.
Play was halted on 82 minutes with Middlesbrough 1-0 up and the incident led to 12 minutes of injury time being added.
And during that time Bielsa's side found an equaliser through Kalvin Philips.
Promising wide man Clarke, who can play on the left or right, has also been eyed by Pep Guardiola's Manchester City, such is the esteem he is held in at Elland Road.
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