Technically the US Marines were already dropped on Guadalcanal, with alomost no supplies and it was a wonder they survived to win the victory over there at all. Most credit to this wonder is going to the japanese, who simply failed to see the importance and the scale of operations on the Solomons, until it was too late (after the US had won the control over Guadalcanal and hold a firm grip on Henderson Field).
If the Japanese had pushed back the first assault in force, the second would still push them out of there, although not likely within a short time. So the capture of Guadalcanal and the Solomons would be delayed by a year or so and the fighting would be worse, more like the one on Iwo Jima, or Okinawa, as the Japanese would certainly have organised some defense by that time.
Another possibility is to have the Allies decide to bypass the Solomons and go directly through the center of the Japanese outer defense ring, via the Mashall and Mariana's, which always had been the strategic idea of the Central Pacific Command of Nimitz. This would certainly not have favoured Mac Arthur and possibly have lead to his demise from the scene early on, but the war possibly would have had been a bit shorter and less bloody, due to the avoiding of the Solomon and possibly the Philippine Campaign. (The last one was a Mac Arthur thing to do and was originally not supported by Nimitz.) Nimitz wanted the Masrhall Islands to launch the bombingcampaign against the Japanese homeislands and bomb the enemy into submission (Which historically was the case already, as Japan was very vulnerable to this sort of bombingraids. Even without the nuclear bombs, Japan was on the point of being breached in august 1945, simply due to the constant bombing of its cities and industries.)