Guy, who won an NCAA title with Virginia in 2019, was picked 55th overall by the New York Knicks in last year's draft. The Knicks then sent him to the Kings.
Guy has primarily played with the G League's Stockton Kings this season.
Guy and his family received an outpouring of condolences on Twitter following the tweet, including ones from the Kings organization, Kings teammate Harrison Barnes and Kings color commentator and ex-player Doug Christie.
According to the latest World Health Organization figures, 1,056,159 people have tested positive for COVID-19 worldwide and 57,206 have died.