Reginaldo sustained an injury to his right thigh in training on Thursday and will miss the first three games of the campaign.
The South American was key to Siena gaining promotion to Italy's top flight last season, scoring seven goals in 30 league apperances.
With Reginaldo out, Siena coach Giuseppe Sannino will be hoping new signing Alberto D'Agostino can make an impact this weekend.
D'Agostino moved to Siena from Udinese and is the only significant reinforcement made by the Tuscan outfit this summer.
Siena captain Simone Vergassola believes his side will face a challenging campaign in their return to Serie A, starting on Sunday.
"We know that the game at Catania will be a tough one for us," said Vergassola. "We all have to prepare ourselves for a very difficult campaign.
"We must be ready to suffer a lot if we are going to defend our status."
Catania, who are coached by Vincenzo Montella, lost just four times at the Angelo Massimino stadium last term.
The Sicilian outfit were boosted in pre-season with Argentina striker Maxi Lopez opting to remain at the club after turning down an offer to join Fiorentina last month.
Lopez scored eight goals in 35 league appearances to help Catania finish 13th in Serie A last term.
"I have remained at Catania because I want to continue to improve," said Lopez. "I want to score more goals than last season and help my team finish as high as possible in the standings."