You don't get time to be a slave to a system in the Premier League unless you're Pep Guardiola. Results and reality take over and the pressure mounts. You need to adapt, muddle through and get results on the job while drilling your system behind the scenes. A football club isn't a lab, it's a living, breathing organism existing in a brutal environment.
It might be grand at a club like Sporting Lisbon to take your time instilling a system, the pressure isn't as great and the opposition isn't as competitive. It isn't grand at Manchester United. The reality is that Manchester United have now lost 2-0 at Wolves - that's Wolves who are in the relegation zone and who have just sacked their manager and replaced him with a fella plucked from Saudi Arabia, on top of a 3-0 home defeat to Bournemouth. These are only the latest in a string of piss poor performances under the new manager.
That isn't good enough.
Klopp at Liverpool was a systems manager and he had some bad setbacks in the early part of his tenure - progress never runs in straight lines - but he got results and performances early to indicate there was a solid basis for optimism. No such basis has yet emerged for Manchester United under Amorim.
Honeymoons don't last long.