Romero was set to form a key part of Conte's plans having started the Italian's first two matches as head coach following the sacking of Nuno Espirito Santo but scans are believed to have shown a muscle tear, with Conte downbeat over his prognosis at a news conference on Wednesday.
Speaking ahead of Tottenham's Premier League home game against Brentford, Conte said: "The news [on Romero] is not positive, not good. Because the injury is serious and I think it will take time before to see him again. Surely next year and we need to check his injury week by week but the injury is serious.
"Now he is trying to recover, he needs a long time ... It's a pity because he's an important player for us but the injury is very serious.
"January, February, I don't know but 2021 is finished to play. We have to wait to recover well but the injury is serious."
December will be a significant test of Tottenham's squad as they face at nine matches before New Year's Day and Conte insisted Steven Bergwijn could help shoulder the goalscoring burden carried by Harry Kane.
Kane is the club's only recognised senior forward at present but Conte believes winger Bergwijn could deputise through the middle if necessary.
Sources have told ESPN that Bergwijn could become available in January after enduring a slow start under Conte but the situation could change if he is able to make an impact during the busy festive period.
"We are talking about a player that has important skills and it was a pity for me, not for the national team because he scored in qualifying for Holland, but I would have liked to have had him during the two weeks of the international break to know him much better," Conte said.
"For sure though he has great quality, quality in one-on-one, fast, quality to beat the man and also to score.
"Now we are working and he is understanding what I want of him. For me, he is one of the striker and he can play in any of the three positions. Surely he is a player that I have to make an important evaluation but he is in my squad and I count on him. Tomorrow he is available for the squad."