Juventus are set to be without Romulo for around four weeks after the midfielder had surgery on a hernia.
The Italian champions face Atletico Madrid in the UEFA Champions League on Wednesday, but travelled to the Spanish capital without Romulo.
A short statement from Juventus confirmed that the 27-year-old "underwent successful bilateral sports hernia surgery in Monaco ... and is expected to be back in action within 30 days".
Speaking at a news conference in Madrid, Juve coach Massimiliano Allegri revealed he had yet to decide who would lead his attack against Diego Simeone's Atletico.
Carlos Tevez seems a certain starter after scoring six goals in his last five appearances, including two as Allegri's men kicked off their Group A campaign with a 2-0 win over Malmo a fortnight ago.
However, it remains to be seen whether Tevez will be paired with Fernando Llorente or Alvaro Morata, the latter having scored his first goal for Juve after replacing his fellow Spaniard midway through the second half of a 3-0 Serie A win over Atalanta on Saturday.
"I still need to choose between Llorente and Morata," said Allegri, who also spoke of a potential recall for defender Martin Caceres.
"Morata did well when he came off the bench on Saturday. He scored his first goal and I was very happy.
"Llorente gives us more of a focal point up front, while Morata drops off and runs into the channels."
Allegri expects a stern test from Atleti, who will be desperate to bounce back after a shock defeat to Olympiacos in their opening group match.
"Atleti reflect the character of their manager. They have amazing psychological strength, stick together and press for 95 minutes," he added.
"Simeone has managed to bring his ideas across and steered them to a La Liga title before missing out on the UEFA Champions League by one minute.
"It will be a difficult game because we're up against an intense, competitive outfit with plenty of quality.
"We need to put in a great performance and try to come away with a positive result."