The Chelsea forward will join the squad on Saturday before a friendly against Honduras in Porto Alegre.
"It is a pleasure to wear the national team shirt once again and represent my country," Willian said on Instagram.
Neymar withdrew from the squad on Thursday after rupturing ligaments in his right ankle near the start of Brazil's 2-0 friendly win over Qatar the night before. Paris Saint-Germain said he has a "severe sprain of the outer lateral ligament."
Neymar is also facing a rape allegation from a woman who says she was assaulted in a Paris hotel on May 15. He denies any wrongdoing.
His agents announced on Friday that he and some of his sponsors have agreed to suspend some advertising campaigns following the allegation.
NR Sports, which holds the rights to Neymar's name and image, said in a statement that no sponsorship contract had been cancelled. The agency did not provide details on what ad campaigns had been cancelled.
"All the partners, for obvious reasons, are alert and aware of the unfolding events," the NR Sports statement said in part.
On Thursday, Nike Inc. said it was "very concerned" about the rape accusation, raising questions about its sponsorship of one of the sport's most famous players.
Willian has made 65 appearances for Brazil and scored eight goals. He last played in a 1-0 friendly victory over Cameroon in November.
His last match for Chelsea was on May 29 when he played in their Europa League final win over Arsenal in Baku, Azerbaijan.
Brazil open the South American tournament next Friday against Bolivia in Sao Paulo.
Information from Reuters and the Associated Press was used in this report.