The Morecambe Missile, a 23-time winner on the Mountain Course, came off his bike in Thursday’s Superbike qualifying at the North West 200 in Northern Ireland.
The initial diagnosis was a fracture to his lower right leg but his Honda Racing team have now confirmed the full extent of McGuinness' injuries with the 45-year-old still in the Royal Victoria Hospital in Belfast.
A statement read: “John has been diagnosed with a compound fracture to his right lower leg, while in theatre on Thursday night it was not possible to plate the leg as initially thought due to the extent of the injury, and tomorrow an external fixator will be fitted.
“In addition, John has also suffered four broken vertebrae and has three broken back ribs, John will wear a cast for up to six weeks in treatment for the vertebrae.
“John will remain at Royal Victoria Hospital for around one week as part of the recuperation process.”
The team, which also includes Guy Martin, have withdrawn from the North West 200 and will next be back on track at Castle Combe in a couple of weeks ahead of the TT.
The 45-year had just set his personal best lap of the week around the 8.9-mile circuit at 118.857mph when he crashed heavily at Primrose Hill halfway through the 45-minute session.
Team manager Jonny Twelvetrees said: “It is a real blow to John and the team, but he’s got an incredible amount of spirit and is in very good hands with the medical staff at the Royal Victoria Hospital.
“We now need to determine what happened and will sit out the Superstock and Superbike races, get back to Louth to analyse John’s bike and re-group ahead of our next test at Castle Combe in a couple of weeks before we head to the Isle of Man.”