Portugal international Ronaldo returned to his homeland earlier this week to spend time with his mother, who suffered a stroke earlier this month.
Due to travel restrictions in and out of Italy to prevent the spread of coronavirus – now categorised as a global pandemic by the World Health Organization (WHO) – Serie A champions Juve are unsure when their star forward will return to Turin.
A statement on the Italian club's official website read: "Cristiano Ronaldo did not train [on Wednesday] and remains in Madeira pending developments related to the current health emergency."
The club later announced that centre-back Daniele Rugani tested positive for coronavirus.
A number of Serie A clubs have suspended training this week, but Juventus' players were again put through their paces midweek, ahead of next Tuesday's Champions League last-16 return leg against Lyon – who lead 1-0.
Gonzalo Higuain trained alone at Juventus Training Centre due to a minor injury and Rodrigo Bentancur withdrew from the session with a thigh complaint.
Maurizio Sarri's Juve beat rivals Inter 2-0 in a behind-closed-doors match last weekend to climb back into top spot in Serie A, one point above Lazio before the suspension of football in the country.