CalMac has been criticised for the arrangements it has put in place after the breakdown of its largest ship.
MV Loch Seaforth was taken off the Ullapool and Stornoway route last week due to an engine failure.
Western Isles Council, Comhairle nan Eilean Siar, said the replacement ferries were too small and could not carry enough freight.
CalMac said it was doing everything to provide services "under extremely challenging circumstances".
The ferry operator has had to move other vessels from their usual routes to help cover sailings between the mainland and the Isle of Lewis, leading to a wider disruption to services.
But the comhairle said it had "deep concerns" about the use of the ferry MV Isle of Arran on the route's freight service.
It said the Isle of Arran could only take four articulated lorries as opposed to the 30 on the Loch Seaforth.
MV Isle of Lewis, another ferry brought in temporarily, can take 13 lorries.
The comhairle said "even a double run of the Arran would appear to fall way short of what is required".
The local authority has been critical of CalMac services previously, claiming the company's "ageing fleet" cannot cope with modern demands.
Leader Roddie Mackay said: "Once again, the people of the Western Isles are being let down by CalMac.
"Using the Isle of Arran highlights CalMac's failure to provide the service for which they have been contracted.
"This latest service reduction comes on top of significant harm done to the Barra service and Barra economy.
"We need a complete overhaul of the cumbersome, ineffective and outdated structures of CMAL and CalMac management. They are not delivering and haven't been for some years."
The Loch Seaforth is at a yard in Greenock for repairs.
Managing director Robbie Drummond said: "We have moved vessels around in order to ensure vital freight services get to and from the Western Isles while the Loch Seaforth is being repaired.
"A combined timetable has been tailored to allow shellfish to be transported from Barra and South Uist to the mainland in time to get to fish markets."
He said the Isle of Arran had been taken away from a busy service to help businesses and that the Isle of Lewis was also carrying freight to maximise available capacity.
Mr Drummond added: "The situation is far from ideal and we are doing everything we can to provide services under extremely challenging circumstances."