It is two years to the day since the Czech midfielder first arrived, initially on loan from Slavia Prague before making his move permanent, and it is hard to overstate the impact he has had both on the squad and club as a whole.
After helping Moyes's side battle the drop in his first half-season, Soucek played a huge role in West Ham’s transformation last term.
The marauding midfielder was a sensation alongside Declan Rice, covering more ground than anyone else on the pitch in the majority of games and proving a real threat in the penalty area, scoring 10 goals across a campaign in which West Ham finished sixth with their highest-ever Premier League points tally and qualified for the Europa League.
It is not just on the pitch where the 26-year-old has made a difference. There has been a big culture shift in east London, and Soucek has played his part there too.
Along with compatriot and former Slavia team-mate Vladimir Coufal, whom Soucek provided a glowing reference for when West Ham were working towards a deal to sign the full-back, he has been setting new standards at the training ground.
The pair have been known to turn up at Rush Green on days off to run and work on their fitness, with Moyes often joking he has to lock them out to aid recovery. Others saw the impact of the extra work and followed suit, Aaron Cresswell among them.
After a barnstorming first full season, however, this campaign has felt a little underwhelming for Soucek, with just three goals this term and far less of an attacking impact on games. Though perhaps it is not all the player’s own doing.
Towards the end of last season, having set the standard for Moyes's energetic side alongside Rice, Soucek had to go it alone for a time while his midfield partner was injured. An intense end to the season spilled into a huge summer in which the Czech Republic reached the quarter-finals of Euro 2020.
As a result, Soucek started the season looking more jaded. A goal on the opening day at Newcastle suggested things would be business as usual, but it would take until December 1, 18 games later, for his tally to reach two for the season.
Soucek has looked off the pace in some games, not his combative self on occasions. The role of the man alongside him plays a part too, however.
Rice has been given more licence to roam by Moyes this season and is flourishing. The England international is commanding on the ball and can dictate games from deep or drive West Ham forward with charging runs.
All that, however, means Soucek must be more reserved, often robbing the Hammers of a dangerous threat arriving late in the box. Rice is more accomplished on the ball, but defenders struggle to handle Soucek.
This is not to say Soucek has been overly poor, he still offers plenty of energy and has gone close with near-misses and disallowed goals which could have given this campaign a very different feel.
He remains a vital cog for West Ham and the club are planning a new deal, likely in the summer. However, two years on from his arrival and with Soucek having shown two very different sides - Moyes faces a quandary.
Does he restrain Rice to bring back the threat of the Czech? Or potentially miss out on more goals to allow his captain to roam free? It is quite the dilemma.