The loss of Cassius was a huge setback that might've sealed the fate of the Liberators. On page 92 of Augustus by Anthony Everitt, we get this:

Then some terrible news arrived. On the same day as the battle, a great sea fight had taken place in the Adriatic. A republican fleet had encountered a convoy conveying two legions to join the triumviral forces. A few transport ships escaped, but then the wind fell and the remainder drifted about in the calm to be rammed or set on fire with easy. The Soldiers were helpless in the face of their destruction. Appian writes:

Some committed suicide when the flames reached them, some jumped aboard the enemy warships, to do or die. Half-burnt ships sailed about for a long time with men on board who were incapacitated by burns or by hunger and thirst. Some men even clung to spars or planks and were washed up on deserted cliffs and beaches.

The disaster was a grim reminder that the republicans controlled the seas. If the triumvirs failed to defeat them by land, they would find it difficult if not impossible to withdraw to Italy; they would be cornered in Greece and would soon run out of supplies. Unsurprisingly, morale among the troops was badly shaken, and Antony and Octavian determined to try to keep the news of the naval catastrophe from Brutus and his men, whom it would excite and reinvigorate.

Essentially, the Liberators had a huge logistical advantage. They could expect supplies by sea, while the triumvirs could expect nearly nothing. On the surface, it looks like Brutus was impatient, and it cost him everything. He shouldn't have rushed to battle so soon.

Ronald Syme argues, on page 205 of The Roman Revolution:

It was not the ghost of Caesar but an incalculable hazard, the loss of Cassius, that brought on the doom of the Republic. Brutus could win a battle but not a campaign. Provoked by the propaganda and the challenges of the Caesarians and impatient of delay, officers and men clamoured that he should try the fortune of battle again. Moreover, eastern princes and their levies were deserting. Brutus gave way at last.

Without Cassius, Brutus couldn't keep his soldiers under control, and he was forced to rush to battle before they deserted him. If you believe Syme, Cassius could've maintained their loyalty where Brutus couldn't have, which would've let the Liberators just starve the triumvirs out.

Sources:

Augustus, Anthony Everitt

The Roman Revolution, Ronald Syme