Eoin Bradley will miss Derry's Ulster final against Donegal and the rest of the season after suffering a cruciate knee ligament injury in training.
Key forward Bradley, superb in Derry's Ulster semi-final win over Armagh, also sustained damage to medial and lateral ligaments and his cartilage.
The extent of the injury was confirmed by a scan on Monday and his absence is a massive blow to Derry.
His brother Paddy is also out for the summer because of a cruciate injury.
Gerard O'Kane is expected to be available for the Ulster decider after missing the win over Armagh because of a thigh injury.
"Gerard is back training. At the moment, he would be 60/40 (to be available," said the Derry boss last Tuesday.
"Whether he makes the team or not is another question. I'm not going to disclose who I have in or out."
O'Kane was replaced by Brian Og McAlary against Armagh and if fit, will be expected to return to the starting line-up although Brennan is giving little away.
"There's a possibility of him (O'Kane) being in the 26," added the Derry manager.