The club have confirmed on their website that the Japan international has suffered a meniscus ligament tear, although the injury is not as serious as first feared
The Japan international was injured during the 1-1 draw with AC Milan last weekend, suffering what was initially hoped to be a simple sprain.
Following further tests carried out by Inter's doctor on Wednesday, the club confirmed on their official website that the 26-year-old has suffered a tear to his meniscus ligaments and was facing a lengthy spell on the sidelines.
However, reports on Wednesday evening suggest that the injury is not as serious as first feared, and the versatile player is likely to be out of action for three to four weeks, meaning he will likely play a further part in the current campaign.
Nagatomo, who joined Andrea Stramaccioni's side from Cesena in 2011, has impressed throughout the past year and reportedly attracted the interest of Real Madrid last month following his displays.
The player signed a new contract at Guiseppe Meazza at the turn of the year, pledging his future to the club until 2016, meaning any interested party would likely have to pay a large sum to tempt him away from Inter.