Los Rojiblancos play host to United at the Estadio Wanda Metropolitano in their Champions League last 16 first leg on February 23.
However, the Argentinian boss faces a host of selection dilemmas ahead of the game, with injuries and suspension impacting his plans.
Alongside a quartet of absentees, Simeone now faces an anxious wait over Lemar.
As per reports from Mundo Deportivo, Lemar was unable to train this weekend, after missing out against Osasuna, as he came into close contact with a positive Covid-19 test in recent days.
Lemar will not be given the all clear until closer to the United game, but Antoine Griezmann is back fit, after being included in the travelling squad at Osasuna.