Why does Sql Server makes this bad query plan choice?
That top being kind of removed from the order by makes is hard for the query optimizer
It does not need to just do a lousy 10 lookups as it need top 10  WHERE [Extent2].[id] IS NULL
With more statistics the query optimizer may get smarter
I know you are using an ORM but give this a try  

SELECT  Top(10) 1 AS [C1],
        [Extent1].[SupplierNumber] AS [SupplierNumber], 
        [Extent1].[ArticleNumber]  AS [ArticleNumber], 
        [Extent1].[Id] AS [Id], 
        [Extent1].[ArticleName] AS [ArticleName]
        FROM [dbo].[SalesEntry] AS [Extent1]
        LEFT OUTER JOIN [dbo].[Article] AS [Extent2]
            ON [Extent1].[ArticleNumber]  = [Extent2].[ArticleNumber]
           AND [Extent1].[SupplierNumber] = [Extent2].[SupplierNumber]
WHERE [Extent2].[id] IS NULL
ORDER BY [Extent1].[SupplierNumber] ASC, [Extent1].[ArticleNumber] ASC