Alexander Isak’s luckless debut season at Liverpool encountered another unwanted wrinkle as he was ruled out of Sunday’s derby against Manchester United with a groin problem which Arne Slot wrote off as a “really small” injury. The most expensive transfer in Premier League history has never recovered from a summer spent on strike to force through the move to Liverpool. It turns out that kicking balls into empty nets at Real Sociedad’s training ground as opposed to an organized preseason with Newcastle United doesn’t provide the best basis for a campaign of Premier League action. Isak could only be sporadically called upon by Arne Slot during his first three months as a Liverpool player as he was eased into first-team action. The cruel blow of misfortune then struck in the form of a broken fibula against Tottenham Hotspur in December. Isak had to wait until April to make his first appearance since Christmas Day but had just begun to find his best form again in recent weeks.…