The surgeons operating on Zlatan Ibrahimovic were so amazed by his level of physique that they want to study his body in further depth, according to agent Mino Raiola.

Ibrahimovic underwent successful surgery at Pittsburgh's University Hospital last week to rectify medial ligament damage in his knee, suffered during Manchester United's Europa League win over Anderlecht last month.

Raiola has now revealed that the ex-Sweden international has started the long rehabilitation process ahead of his likely return to action in 2018, although there is hope of an earlier comeback due to the nature of his fitness.

"Zlatan has started rehabilitation at the hospital and will be monitored by Dr. Fu and Dr. Musahl until he is restored," he told Expressen.se. "His knee is so strong that the doctors said they had never seen anything like it.

"He has a knee that it is almost impossible for a football player with a 20-year career to have. It was quite clean, there was no harm in it. Zlatan is so strong that the doctor wants him back after his career to research on him.

"They work in the world's best research institutions for the knees and ligaments. They research a lot on the subject and that is why they are better than everyone else. And the doctors want back Zlatan to do research on him. So after Zlatan's career, we will go back and open him up again to do research on his ligaments."

Ibrahimovic has still yet to make a decision over his future beyond the summer, with his Old Trafford contract due to expire next month.