The 35-year-old had joined back up with his team-mates ahead of his first full campaign with the Brazilian side since returning on a free transfer from Lazio over the summer.
However, tests conducted by the club's medical staff ahead of those preparations getting underway showed an "alteration of the cardiac rhythm".
Consequently, Lucas has been pulled out of all physical training until further tests can be completed.
The former Brazil international midfielder confirmed the news on Twitter on Tuesday.
Replying to his tweet, Liverpool FC wrote: "We’re all with you Lucas."
And his former Lazio teammate Felipe Caicedo said: "Good recovery brother, you will come back stronger."