Wait goes on for return of Huddersfield Town's record signing
The £11.5m summer arrival has only recently returned to the squad after missing five games with a heel problem.
Mounie has come off the bench in the last two games but struggled to make any impact.
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Hide Ad“He was out for a longer time than we expected,” said Wagner, who says Rajiv Van la Parra is fit to face West Bromwich Albion on Saturday.
“We made some tests with him at the start of the week and, fitness-wise, he is on a good level but not a level where he was before the injury.
“At the start (of the season), he was one of the top two in the group, fitness-wise. But he is not back at that level yet, even if he is still at a good level compared to the group.”
Mounie, who netted twice in the opening day win at Crystal Palace, is due to join up with Benin after the West Brom game for a friendly double-header against Tanzania and Congo.
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Hide AdWagner added: “The international break will suit him. He will be away with Benin and have two further opportunities to get minutes in his legs for the national team.
“I think he needs this time. He has had a good week so far, he has now been back in full training two weeks. We are happy to have him back but he is not yet at the level he was before. That is not a surprise for me.”