Maybe.
The two forces were, of course, separated by the Brandywine River. Knyphausen with over 6,000 men demonstrated against Washington’s main position, while Cornwallis with over 8,000 went upstream to cross and outflank.
Now, if Sullivan had been able to get into position a bit earlier, *maybe* they could’ve given Cornwallis a repulse. But with the strength and discipline of his division, and his opponents having had to hurriedly redeploy to meet him, plus which the Continentals were by no means as well trained as they later were, I doubt they’d have been able to utterly overthrow him.
And while the troops left in the main position could perhaps have given Knyphausen a bloody nose, it is highly unlikely they could’ve smashed him, since they didn’t outnumber him significantly and they’d have had to force their way across the River to finalize his defeat.
Which means, in my judgement, the absolute best they could’ve plausibly done was stymie both attacks, and a stalemate would’ve necessitated Cornwallis’ recrossing the River to get the army back together. Back to Square One. But I doubt the Americans could’ve inflicted more than 1-2,000 casualties, since totally demolishing either British division just wasn’t happening.
At this point, if Howe doesn’t lose his nerve, his army can probably try again. Now begins another bout of move and counter-move, as Howe tries either to fight through Washington’s position or to out-maneuver him and force him to fall back. If the Americans counter well enough, they can perhaps “run out the clock”; it’s already September and if things drag out Howe will have to go home to winter quarters.
But in order for this to happen, Washington’s army has to perform noticeably better in several ways during the maneuver phase following Brandywine. They have to keep better-informed about British movements than they did historically. They have to move troops more quickly and accurately than they seem to have been capable of during this period, and they have to fight better than they did historically at this point in the war.
If Washington beats Howe at Brandywine, I think the odds are the British still get Philadelphia, just a bit later.
Unless Howe loses his nerve. I dunno how likely that’d be.
I’m no expert on the campaign, but this is how I see it.