It’s up to Batman and Wonder Woman to form the Justice League in Superman’s absence. The Man of Steel apparently died after the fight with Doomsday, and that was also the last time we saw Batman and Wonder Woman. Inspired by Superman's sacrifice, and knowing that darker days were ahead, Bruce Wayne and Diana Prince went in search help from their fellow superheroes.

We saw the impact Superman's death had on the heroes in those final moments of Batman v Superman: Dawn of Justice. But Bruce Wayne found hope again, and Diana Prince decided to take up her mantle as a superheroine and become involved in Man’s battles.

Ben Affleck told US Weekly that Batman is a loner, but he makes an effort to get along with the metahumans. He went on to say that Bruce Wayne could even become a mentor to Flash:

In Batman v Superman, he was at the end of his rope. But in Justice League, he's finding hope again. He has to open up and play well with others. He knows he needs them. He's sort of the ultimate loner, but he's really trying to make it work and develop a good relationship with all of them. He might be a mentor to The Flash.

Due to her age, Wonder Woman is more experienced than the other Justice League members. In addition, she also managed to defeat Ares during World War I. But Gal Gadot commented that she shared Batman’s understanding of the loss of Superman:

Since they've fought together, they know each other's strengths. They also understand the hope Superman stood for. That was lost [when he died], so they feel that more than the others.

Apparently, the founders of the DCEU's Justice League haven't completely healed yet. The big question is whether they ever will, given the widespread speculation that Superman will turn evil when he does appear.