The 21-year-old, capped 41 times by Slovenia, is set to lead the attack alongside Cunha and Mbeumo as new manager Rben Amorim seeks to improve on United's 44 goals in 38 league games last season.
West Ham United marked their first match at the Olympic Stadium with a 3-0 win over Slovenian side NK Domzale, Cheikhou Kouyate scoring twice to help secure a place in the Europa League playoff round.