I’m in favor of dropping both Gana and Onana for the Palace match, and starting Garner and Davies or Doucouré. Use Davies as DCM, Garner and Iwobi as ACMs. If Doucouré rather than Davies, I’m not sure Doucouré is capable of sticking to a sitter/DCM role, as he’s a wanderer. So if it were to be Doucouré and Garner, perhaps Garner sits and protects, allowing Doucouré and Iwobi to attack. Both Iwobi and Doucouré are energetic enough to defend, too.
Re Gana/Onana, I’m a bit more hopeful that Onana will come good as the season progresses. I worry that Gana thinks he’s a box-to-box player, and even more worried that Lampard thinks that, too.
Although Gana seems somewhat less likely than in his previous years at Everton to run all over the pitch, he still ventures too far forward, even though as an attacker he has few skills, and when too far forward is out of position as DCM. He should sit and protect the back 4, only rarely venturing forward much past midfield. He needs to do fewer things, and concentrate a helluva lot more on taking few risks and protecting the back 4, not to mention protecting the ball.