No definitive timescale has been put on Armitage's absence but, with the Exiles' opening European encounter now less than a month away, the 24-year-old could well be forced to watch from the sidelines.
Armitage suffered the setback after just 21 minutes of Irish's league defeat at Bath last weekend, with the initial prognosis being a torn calf miuscle.
However, scans have since been confirmed that the England international has a minor fracture to the smaller of the two shin bones.
Toby Booth's men begin their European campaign with a mouth-watering clash with Munster at the Madejski Stadium on October 9 before traveling to Swansea to face the Ospreys a week later.
Also in what is being described as a 'pool of death' are last year's beaten Amlin Challenge Cup finalists RC Toulon.
Irish started last season's continental adventure with a stunning win over defending champions Leinster in Dublin but a brace of defeats to the Scarlets and a home draw with the holders saw their challenge end at the group stages.