Lampard admits the midfielder, who was set to return to PSV Eindhoven last summer, has suffered a setback in his recovery.
He stated: “He is injured for a longer period.
"Unfortunately, there were some complications with his surgery. We do not want to set a precise time for his recovery, but let him rehabilitate calmly. He is very diligent and his attitude could not be better.
"He works extremely hard every hour of the day. He is positive, despite the fact that I am convinced that it is very difficult for him."