Sexwale, who was an anti-apartheid campaigner and former political prisoner on Robben Island, is against a boycott but expressed concern in an interview with The Associated Press about the growing number of racist incidents in the Russian league.
"There is a threat black players will say they are not going to Russia (for the World Cup) — we can't have that," Sexwale said Tuesday on the sidelines of the Doha Goals conference. "I am talking as a FIFA person and a citizen of the world — it can't go that far. ... Once these things start and you don't act as leaders, these things snowball."
Although UEFA has punished Russian clubs for racism at Champions League matches in recent years, the national federation has appeared less willing to tackle abuse.
Sexwale is urging his "personal friend" Putin to intervene.
"Show that leadership, be the Putin the world knows, be tough," said Sexwale, a former South African government minister who is an adviser to FIFA's anti-racism task force. "Failure to do so, we could be talking something different about the 2018 World Cup ... you will have people saying they will not go to Russia."
The Russian Football Union has been criticized for taking no action against FC Rostov coach Igor Gamula for saying last week he wouldn't sign a defender from Cameroon because the club has "enough dark-skinned players, we've got six of the things." Gamula apologized after the agent of Rostov defender Siyanda Xulu, a South Africa international, said that five African players on the team threatened to sit out Monday's training.
Sexwale said an apology alone wasn't enough, pointing to how the NBA forced Donald Sterling to sell the Los Angeles Clippers and banned him for life over racism.
South African Football Association president Danny Jordaan wrote to FIFA President Sepp Blatter on Monday "expressing concerns," according to Sexwale.
"For the Russian federation to be seen to be serious we need them to take stern action. The world took stern action against South Africa (over apartheid), it was expelled from FIFA," Sexwale said, stopping short of calling for Russia to face the same sanction.
CSKA Moscow has been forced to play Champions League games without fans this season because of fan abuse toward black players, although Sports Minister Vitaly Mutko maintained last month that Russia has no major racism problem, saying: "I don't know what there is to be frightened."
But Sexwale said of Moscow: "There are certain parts if you are my colour it's unsafe ... people are scared of going to Moscow."
Sexwale wrote to South Africa's ambassador to Moscow on Monday to "caution that this is very dangerous."
"If these ultra-ring elements have their way they are threatening the World Cup," Sexwale said. "The authorities have got to come down on the perpetrators, otherwise the authority is now going to be boycotted by the victim because people will lose confidence in the Russian federation."
Claudio Sulser, the head of FIFA's disciplinary committee, said in Doha that the organization would deal with any racism at the World Cup.
"It's very critical," Sulser said. "This is a problem of the society."
Rob Harris can be followed at www.twitter.com/RobHarris