It's absolutely impossible to say. The only way this can be valued is by seeing a copy of the full medical report and full prognosis.
Damages for pain suffering and loss of amenity (general damages) are valued by a) using the JSB guidelines and b) comparison with previous cases, but I would always want to view all of the medical evidence before nailing down a figure. it may well be best to wait until after the knee replacement since only then can a better prognosis be given. In the meantime an interim payment can be applied for.
In addition, there will be special damages - these will be loss of earnings, and actual financial loss (ie visits to doctor, physio etc). Furthermore, there may well be a claim for loss of future earnings. But this will be worked out on type of career, salary etc.
The solicitor should seek advice of counsel who will be able to place a much more precise value since he has all the papers in front of him.