There was "a" lantern in the Justice League flashback. It would be easy to write another in wether it be a Hal Jordan lantern or any other member of the lantern corps.
DC must still be hurting from the Green Lantern flop. I don't think they should bring Ryan Reynolds back anyway. His comic meanderings are over. He is Deadpool now just as the other meanderer is now Cap. Too much success in those roles not to be locked in.