The 18-year-old was withdrawn at half-time when he complained of feeling unwell before being taken away from the dugout on a stretcher.
Leeds issued a statement that he was “responsive” in hospital but fans were understandably concerned for his welfare.
And Clarke, who was released on Saturday evening, took to Twitter to thanks fans for their messages.
“Would like to thank everyone for the support and love they have shown, massively appreciated,” he said.
Clarke’s agent Ian Harte – the former Leeds left-back – revealed on Saturday evening that his client was on the mend.
Harte wrote on Twitter: “Big thanks to @Boro medical team and a special mention to @LUFC Rob Price for today!
“Jack has left the hospital now happy and smiling. Hopefully back on the pitch very soon.
“And also a big thanks to everyone involved with James Cook hospital the staff have been brilliant!!!”
Leeds chief executive Angus Kinnear revealed this morning that Clarke will be given the week off from training to recover.
He said: “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.”