The goal was Sesko’s 10th league goal of the campaign.
Tomas Soucek pulled one back for West Ham in the 49th minute when he guided home Malick Diouf’s pass from close range.
Bukayo Saka put Arsenal ahead only five minutes into the encounter.
Reduced to ten men, the Blues were immediately put under pressure and United capitalised.
Both clubs have now finalised the deal, with United determined to secure the forward ahead of their pre-season tour in the United States.
Champions Liverpool face Bournemouth in the season’s opening match as league organisers release the matchday schedules for the upcoming season.
Arsenal moved second on the league standings after the Gunners secured a dominant 5-2 away win against West Ham United in their Premier League tie at the London Stadium on Saturday
Arsenal has temporarily moved to top of the Premier League table after the Gunners secured a late 2-1 win over Brentford at the Emirates Stadium.
Champions Manchester City are now one point ahead of Liverpool at the top of the Premier League table after Pep Guardiola’s side defeated Brentford 1-0.
Salah scored Liverpool’s third goal in the 68th minute.
