Why dont you think Witsel "didnt cut the mustard at benfica"?
Are you asking me why I hold that belief or agreeing with the view and then asking me the specifics of why he didn't meet the standard?
To answer both questions simultaneously, it's based on a good friend's perspective (who actually is Portuguese and a Benfica fan) on Witsel.
His assessment was that while he had bags of ability and on his day could shine above many others, more often that not he looked fairly average.
Also, his top performances were apparently against opponents of a much lower calibre. He then went to the Russian league which says a lot.
Honestly I've not personally seen much of him bar international games and CL football, and what I have seen he's looked pretty decent.
However, I also highly respect my friend's perspective on players so I'm open to be convinced he'd be good enough in the Premier League.