The fleet had probably been fighting the Borg for days
before the Enterprise arrives. The battle had limped back to Earth's own doorstep, and countless how many ships have been left in the Cube's wake. The couple guys left are probably a small rabble compared to the fleet that actually engaged the Borg off-screen earlier in the battle. So they probably aren't likely to start picking nits when Enterprise (a ship which had been deliberately left clear from the conflict, so therefore probably in better battle condition than most of 'em by that point) flies in and takes the place of the destroyed flagship.