The vessel will provide an additional 10,000 passenger spaces for the festival later this month.
The Turkish-built MV Isle of Islay has been unable to use its bow doors since mid-May.
CMAL has taken legal ownership of three ships as the shipyard faces economic challenges.
The crew of the 79-year-old Waverley offered its assistance to get passengers back to Oban on Tuesday.
CalMac says the past few weeks were "immensely challenging" due to the number of boats out of action.
A change in ferries had led to the situation on Tiree.
Petrol purchases are being capped at £25 as fuel levels are running low.
The ferry will resume service on the route between Oban and Craignure where it is filling in for the MV Isle of Mull.
Ferguson Marine said it was unable to bid to build a replacement for MV Lord of the Isles because of eligibility criteria.
