The South African will spend the entire season on the sidelines after tearing his anterior cruciate ligament at the weekend.
Des Fountain suffered the setback in the first minute of Saturday's narrow defeat to Ulster, one of the three sides Aironi will face in pool stages of the Heineken Cup.
"We are very sorry about Dylan," said Aironi chief executive Franco Tonni.
"With this injury, we lose a player with great qualities, both on and off the field.
"We are confident Dylan will recover, however. We expect him to have recovered by the end of June, when we will consider his possible return to Aironi".
Aironi travel to Belfast to face Ulster in their tournament debut on Friday, October 8, before hosting 1998 European Champions and 2008 Amlin Challenge Cup winners Bath Rugby the following Saturday.
Also in Pool 4 are last season's beaten Heineken Cup finalists Biarritz Olympique Pays Basque.
Aironi were only formed this summer and have so far begun life in the Magners League with a promising performance in defeat at Munster and a 22-15 reverse against Ulster last Saturday.