"The medical team will probably have arranged some imaging of the injury. This would usually involve putting on an air cast boot and making him non-weight bearing on crutches," Lewin said.
"They will arrange an MRI scan and possibly a dynamic ultrasound to determine if the tendon is just inflamed, partially torn or ruptured.
"Obviously, the treatment will depend on the severity of the injury but he could be sidelined for as little as two to three weeks if it turns out to be only an inflammation But looking at the mechanism, I would suspect a rupture."
Yarmolenko, who scored twice against Everton last month and assisted one of the Hammers' goal in the recent win over Manchester United, suffered an achilles injury while with Borussia Dortmund last season that ruled him out of action for 12 games between January and April. As things stand, it could be April once more before the 28-year-old is back in action.