A knee injury for Maddison will see the midfielder miss much of the season.
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It's been confirmed the England international will undergo ACL surgery in the coming days.
Frank told football.london: "I think that's devastating news for the team and for Maddison.
"No doubt that we will miss him, and he will miss us. So right now, we just need to get through the first difficult spell until the surgery, and then in a few weeks after that there will be a little light that turns on and then he can work forward."