You will just have a little less flavor. I doubt it will make a huge deal. You could still boil up some concentrated DME in 1-2 cups of water and add that if you are really worried about it. But I would just let it ride.
BTW, I make all my recipes to 5.75 gallons in a 6.5 gal bucket. Although I do add enough fermentables to compensate. I lose between 0.5-1 gallons of beer in trub loss and don't want to get shorted the finished brew.
Primary #1: Empty #2: Black Currant Wine
Secondary #1: Berry Rhubarb Wine #2: Empty
Kegged: Cascadian Dark Ale
Bottles: Spruced Winter Warmer, Surly Bender clone, Big 50 Barleywine, Framboise Lambic, Dark Belgian Strong, Kicked by a Moose Scotch Ale, Russian Imperial Stout, Sparkling Elderflower Wine, Barolo Wine, On Deck: Orange Pale Ale IV, Hidden Lake Nut Brown, 2H IPA III