Unwanted striker M'Baye Niang has issued AC Milan with a medical certificate citing "psychological problems" to justify missing training after rejecting an offer to join Spartak Moscow.

Niang is surplus to requirements at the Serie A giants following their signing of Andre Silva from Porto and Franck Kessie from Atalanta earlier this summer.

Spartak were reportedly on the 22-year-old Frenchman's trail but Niang, who has been loaned out three times since joining Milan in 2012, rejected the offer to join the Russian Premier League side.

Milan coach Vincenzo Montella hit out at Niang's attitude in training earlier this week, to which the Frenchman replied: "My stress is linked to what's going on off the pitch, it has nothing to do with Milan."

Niang has been absent from training in recent days and his agent Mino Raiola, speaking to Sky Sport on Thursday, said the decision should be respected.

"Montella said the lad wasn't training because his head wasn't in it, and the medical certificate confirms it, so Montella is right," Raiola said.

As far as any future move for Niang is concerned, Raiola added: "We're looking for a solution that suits everybody."

Niang joined the seven-time European champions in 2012, but after failing to cement a regular starting place he spent loan spells at Montpellier (2014), Genoa (2015) and Watford (2017).