Xjyuk

At the end of Avengers: Endgame, Sam Wilson is chosen as the

next Captain America.

I understand WHY he is chosen. Bucky is mentally damaged and can be easily manipulated by Hydra. But Sam is a mere mortal.

He wasn't injected with any kind of serum like Steve. Unless they plan to inject Sam with some hormones, what makes him the next Cap?

Captain America isn’t about being a super soldier and being able to kick ass because of how powerful you are. It is about standing up for what is right, helping out everyone no matter the cost. Incidentally this is also why Steve in Avengers: Endgame

Is worthy of Mjolnir.

This is in part why Sam was chosen over Bucky because he shares Steve’s ideals and morals and wants to do what’s right.

And even if we look past that, Sam as the Falcon can hold his own quite well being able to fly and all has some serious advantages. Add the shield in to the mix and he has some serious power as a fighter. He doesn’t necessarily need the super soldier serum, he is an Avenger without it after all.

In the first Captain America movie, a frail Steve Rogers also asked why he was the one being chosen. Yes, I know your question is technically not about the why, but the answer Erskine gave him still stands...

ROGERS: Can I ask a question?

ERSKINE: Just one?

ROGERS: Why me? [...]

ERSKINE: [...] But more important, the man. The serum amplifies everything that is inside, so good becomes great; bad becomes worse. This is why you were chosen. Because the strong man who has known power all his life, may lose respect for that power, but a weak man knows the value of strength, and knows... compassion.

ROGERS: Thanks. I think.

ERSKINE: [Gesturing toward the wine] Get it, get it. Whatever happens tomorrow, you must promise me one thing. That you will stay who you are, not a perfect soldier, but a good man.

Sam Wilson, while not necessarily a "weak man", does qualify for the role - see his "good actions", good heart etc throughout the MCU. As @TheLethalCarrot noted, he's a decent fighter to begin with - and comics-wise, Sam Wilson as Captain America in All-New Captan America was holding his own nicely (with no serum involved, IIRC).

And in case he needs the occasional amp-up, he's friends with gods, superpowered people, the Sorcerer Supreme, Professor Hulk and all eight of his PHDs - he'll be fine.

Sam Wilson became Captain America in the comics. While he did have a superpower there, that superpower was telepathy of and control of birds so it's not exactly a substitute for a super soldier serum. He seems to have done perfectly fine there.

In the MCU, there are numerous other superheroes that are just regular people, who just have very good training or technologically advanced equipment: Black Widow, Hawkeye, Iron Man, Ant Man, etc. Sam Wilson has the equipment (the wings and drone) and the training (military).

It's possible that the Disney+ Series Falcon & Winter Soldier will cover how he does in his new role, but for now we have too little information about it to even say when it takes place.