The 29-year-old Strudwick injured his ankle when Irish beat Newcastle 20-0 on Sunday.
It was initialy feared he might have broken his ankle, but X-rays revealed no fractures. Futher scans were needed, which brought to light torn ligaments.
Strudwick will have his ankle in plaster for between four to six weeks and he will be sidelined for three months in total.
Irish is currently third from botom in the Zurich Premiership standings – on 22 points, the same as Bath, with bottom placed Newcastle on 16 points.
Apart from the remainder of the Zurich Premiership, the South African lock forward will also miss the Exiles' remaining Heineken Cup group matches against Edinburgh and Toulouse, as well as the Powergen Cup quarter-final against Rotherham later this month.