"Good enough" depends on what you're aiming for. If you're just looking to improve on what we have then I guess Fellaini is good enough, as is any player who's better than Cleverly (our 2nd best cm). If you're looking for a player that makes our cm as good as it was the last time we won the CL then no, I don't think Fellaini is "good enough". If you're looking for a player that's good enough to get into any of the best teams in Europe then no, Fellaini isn't near good enough. If you want Fellaini as a squad player then that's fine, if you're expecting him to bring our midfield to the same level as our rivals for the CL then you'll be disappointed, imo.
As for what should be good enough for us, that's simple: we should have one of the best midfields in Europe. We claim to be one of the biggest and best clubs in Europe so we should be ambitious enough to say that any aspect of our team that doesn't match up to our rivals in Europe needs improving. That doesn't mean Xavi or Iniesta but it does mean players better than Fellaini in our 1st team.