He has only played twice all season, having also missed the start of the season with a back problem.
The 27-year-old has now returned to light training following surgery.
"Diaf is essentially at the start of his own personal pre-season training schedule," Hammers medical boss Stijn Vandenbroucke said.
"When players return from their off-season, they will go through four to five weeks of pre-season work to build up their strength and fitness.
"After undergoing back surgery and three months of rehab, a player needs time to reach sufficient fitness levels before we can contemplate making him available for selection in the Premier League."
Senegal international Sakho, who was due to join West Brom for £15m before his summer injury, missed this season's Africa Cup of Nations.

Kamdjo, 25, played in only five games for Forest Green last season, but made 33 appearances on loan at Boreham Wood.
He started his career with Reading, and had spells at Barnet and Salisbury before joining Rovers in 2014.
York were relegated from League Two last season after winning only seven league games.

A Â£100,000 reward is on offer for information leading to the conviction of the killers of 14-year-old Charlene Downes or the recovery of her body.
An appeal will be made on the big screen and inside programmes during the team's match against Brighton at Bloomfield Road on Saturday.
Charlene disappeared near Blackpool's North Pier on 1 November 2003.
Police believe she was one of a number of girls who were groomed for sex.
Lancashire Police and Crime Commissioner Clive Grunshaw said: "Someone out there knows something about what happened to Charlene.
"Detectives believe the answer lies in Blackpool - and it could lie at Bloomfield Road on Saturday."
Det Supt Andy Webster said: "We have never given up our search for information about Charlene, who we believe was murdered more than a decade ago.
"We appreciate that a considerable amount of time has passed since Charlene went missing, but we would still encourage anyone with information at all to come forward and speak to us."
In 2008, a judge ordered a man to be cleared of Charlene's murder, after "grave doubts" were raised around evidence during his second trial. The jury in his first trial failed to reach a verdict.
The following year, the Independent Police Complaints Commission (IPCC) said police errors in gathering evidence had contributed to the collapse of the retrial.
The IPCC investigation concluded the investigating team was guilty of a strategic and tactical failure in the management of the material and several officers were disciplined, with one being forced to resign in 2011.
The resignation order was overturned by a Police Arbitration Tribunal in 2012.
West Ham striker Diafra Sakho is unlikely to play again this season because of the back injury he suffered in November.
[NEXT_CONCEPT]
York City have signed midfielder Clovis Kamdjo on a two-year contract following his release by National League rivals Forest Green Rovers in May.
[NEXT_CONCEPT]
Blackpool football fans are being asked to help solve the murder of a teenager who disappeared over a decade ago.