However, Mariner is keen to keep the 24-year-old at Home Park and is optimistic Fulham will agree to an extension.
Mariner told The Herald: "We are trying to extend David's loan and it's looking extremely positive.
"He has fitted into the squad very well. He's an amiable lad and he works very hard at his game.
"I think he has proved to everyone that he's very commanding in the box.
"He's a young goalkeeper so he has still got things to learn. He's by no means the finished article and David understands that."
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Source: Team Talk
Source: Team Talk