Talks about a new deal collapsed due to the players high wage demands. Nani, who currently earns around £90k weekly, has demanded nearly £120k-a-week. However, the Portuguese international is ready to cut-down his wages in order to save his career with United.
"The news about Nani is true," said Zenit coach Luciano Spalletti.
Nani was criticised for his poor performance during the loss against Everton on the opening day while he was ignored on the bench in United's win over Fulham, raising doubts on his future with the Red Devils.
Manager Alex Ferguson will also look to offload few players after spending nearly £50 million including Robin van Persie and Shinji Kagawa this summer. In line for late transfers are striker Javier Hernandez, out-of-favour Dimitar Berbatov and Anderson.
With a £25 million offer from Zenit for Nani, United won't be reluctant to sell, who are also planning to give away midfielder Anderson for a reported fee of £15m.
Meanwhile, Tottenham Hotspur are planning to bid for the 24-year-old Brazilian in case they fail to sign top target Joao Moutinho. But the Porto star will cost a mammoth £30m while Spurs are also eyeing Brazil's £20m-rated striker Leandro Damiao.
Nani faces stiff competition from teammates winger Ashley Young, Antonio Valencia and veteran Ryan Giggs. England international Young was given the start against Fulham while Nani's poor show at Euro 2012 and a drop in form during the latter half of the previous season has forced Ferguson to consider his options.