In the grand scheme of things, days like today yes are unavoidable. It is the reason why no team, even the best in the world, goes perfect through a season. It is why long winning runs are very hard to do - and we were on 9 wins in all competitions. Ignore the game in isolation. Is Man United, with our current group of attackers (aka just Rashford) 6 months into EtH, capable of going on long winning runs? Capable of challenging for the title? It's not because of mistakes in 1 off games, it's because we simply don't have the players for that yet and it's something that simply getting a better striker (very low bar) will sort out, as will just our other young wingers developing more (Antony, Garnacho, Sancho for example are all young with potential).
Looking at the game in isolation and saying we have to learn a lesson isn't the correct way to analyze the game, because we've had similar games multiple times this season. Some of them we've won, some we've drawn, but they're games where we played well for a good chunk, but didn't get the 2nd goal (as we are a 1 man attack), and then you rely on some luck to see out the game. Leicester, Southampton, Newcastle, west ham, wolves, and now palace... All very similar games. All good for most of it, had a small wobble period, but all games we only scored 1 goal in (or 0) and on another day, they could've scored a 35 yard screamer out of nothing to turn the result. We need less of these types of games is the lesson, because some will inevitably go this way. The only way to have fewer of these games is by getting that 2-0 goal earlier, and that only happens with a striker.
It doesn't drastically affect our outlook. Our outlook is 3rd this season. We were never going to be title winners this year. A draw here, and even if we lose to Arsenal on the weekend, doesn't change that, as even with a loss, we will likely be still 6 points clear of 5th.