Ibrahimovic is in United's squad for the clash with Newcastle United - for the first time since April knee surgery.
Walden, who is knows Ibra's surgeon Freddie Fu and works for IFK Kristianstad, admits the striker's return is ahead of normal schedule.
He told Expressen: "In general, the time before reset is between six and nine months. 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."
Walden added: "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."