Okwonkwo might have blown his chance in pre-season. He was a nervous wreck. Hopefully he bounces back but I doubt we'll be relying on him or Hein as second choice just yet.If Leno leaves our backups would be Karl Hein and Arthur Okonkwo. They're both young, but have a lot of quality.
Hein is already Estonia's #1, so he's gaining a lot of experience at a high level. Okonkwo appears to slightly ahead of Hein in the GK pecking order, but they're at about the same level. Right now Hein and Okonkwo are fighting to be the third choice GK. Hein is on the bench against Wimbledon, so he might be ahead of Okonkwo now.
If these two get half a season of experience with the first team this year, they could be ready to backup Ramsdale if Leno leaves. My guess is that Arteta is banking on these two to develop a lot over the next couple of months if we're going to let Leno leave.