The Czech midfielder has now scored in his last five matches for club and country after an 89th-minute volley earned a scratchy 1-0 win.
Around 320 hardy West Ham fans made the 2,000-mile journey out to the Balkans, including a 100-mile trek up the motorway from Belgrade to the TSC Arena.
But the youngster barely got a sniff of the ball as West Ham struggled to rouse themselves in the sleepy Serbian town.
The hosts came into the encounter on the back of a 4-0 win over Super League leaders Partizan at the weekend, and they made a confident start with Uros Milovanovic forcing an early save from Lukasz Fabianski.
Then an error from Aaron Cresswell, handed a rare start in one of seven changes from the side which snatched a late win at Burnley thanks to Soucek’s goal, gifted Aleksandar Cirkovic a shooting opportunity with Fabianski saving again.
West Ham registered their first shot on target after half an hour, a tame effort from Said Benrahma on his 150th West Ham appearance which was easily dealt with by Veljko Ilic.
But it was a mind-numbing first half neatly summed up by former player Joe Cole, on TNT Sports, as “dross” and “appalling”.
Boss David Moyes turned to Danny Ings and the lesser-spotted Maxwel Cornet just after the hour, replacing Benrahma and Mubama, in a bid to inject some life into his unconvincing side.
And as the game ticked into stoppage tie Cornet crossed from the left and Soucek hammered in a volley to win it.
West Ham have enjoyed a thrilling run in Europe over the past three seasons, including their historic Europa Conference League final triumph in Prague.
Although this was not one which will live long in the memory it was an 18th win in 20 matches in Europe, job done for Moyes and his side.
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