It made sense when we were sweeping all before us. No need or point in disrupting a winning system. But we're currently very close to broken and we're clearly struggling fitness wise. Klopp's reluctance to use him reflects badly on either Klopp or Kostas - we can't know the answer to that question because we're not seeing training.
Klopp's usual tactic is to ease players in very slowly, get them used to the system, and build their confidence.
In a clusterfuck of a season like this, that caution can backfire. These players don't get the bedding in time, they're expected to make an immediate impact, and it seems Jurgen would rather risk the fitness of established players rather than the self confidence of new players, or make the team even more disjointed than it already is.
Of course, he had to in the end, and the result has been a rather piecemeal approach. I don't think it's a coincidence that the players who have drawn the biggest questions or criticisms over the past 14 months - Minamino, Kostas, Kabak, and even Thiago to a lesser extent - have all been signed during the pandemic.