I'm not sure what's broken his roster status (the other World Cup players seem to have returned normally), but there's a simple fix:
Shop Mantha (right-click, Shop Player), then accept one of the offers for him. After the trade completes, immediately edit both him and the traded player back to their original teams. That fixes his roster status and you'll be able to use him normally again (which inclines me to think something other than international play did this, because a trade wouldn't change that status, but I can't tell what the actual cause was.)
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