Swedish knee specialist Markus Waldén has warned that Zlatan Ibrahimovic is not yet out of trouble having completed his recovery from anterior cruciate ligament damage.
The enigmatic forward suffered the injury in a Europa League clash with Anderlecht in April, and was expected to miss the remainder of the year.
But his determination to recover saw him make an earlier than expected comeback in Manchester United's 4-1 win over Newcastle on Saturday.
Walden, however, has stressed that Ibrahimovic must not overburden himself as he continues the process of returning to full fitness.
"In general, the time before reset is between six and nine months," he told Expressen. "The trend is to approach nine months more than six.
"The average time for a professional player is seven months to the first training session and eight months to the first match.
"The ligament may break again. The risk is the greatest in the first year after operating. At least half, maybe three out of four operated ligaments that break apart do so within one and a half years after surgery."
Ibrahimovic himself revealed after his cameo against Newcastle that the injury was more severe than many realised.
“If people knew the real injury, they would be in shock that I was even playing. It was more than the knee but I will keep it personal – that’s why I choose not to talk of it,” he said, quoted by the Guardian.
“In my recovery I have not rushed anything; I have been out for six and half months. I have been following protocol and my schedule day by day. The only secret has been working hard. Those close to me know what I have been doing. I was working five, six hours a day.
“When it happened I said giving up was not an option. My only focus was on coming back and coming back better. I know it’s my head playing, not my knee; my knee has to follow. With my experience and career I know what I have to do but I feel good.
"I am very proud of the moment to play again and for everyone around me because I wouldn’t have done it alone. Thanks to the supporters for keeping me motivated, the coach who’s trusted me, the club that believes in me and my family obviously.”