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.