By Grey Whitebloom Danish champions FC Copenhagen kick off the Champions League knockout stages with a daunting visit from the competition’s holders Manchester City on Tuesday night. Copenhagen have not played a competitive fixture since defeating Galatasaray in their final Champions League group game two months ago. The Danish top flight does not resume until next weekend but Copenhagen have tried to limit the rust with a series of friendlies in the new year. City, by comparison, have been crowned world champions and clicked into ominous form since the club’s last European fixture. Pep Guardi…