The game was the one Moisés Caicedo would want to forget in earnest after the Ecuador international gave away a penalty late in the match.
Said Benrahma, however, broke the deadlock, firing the Hammers ahead in the 62nd minute from the penalty spot.
Roma, Sevilla, West Ham and Fiorentina book tickets for European football showdowns.
The result leaves Arsenal, who are currently on 74 points, only four points ahead of defending champions Manchester City who still have a game in hand.
With 40 points in 15 games, Arsenal are now seven points ahead of Newcastle, who earlier on Monday beat Leicester City.
Uruguay striker Nunez was missing in the Nottingham Forest game due to a hamstring strain from Wednesday’s game with West Ham.
The 21-year-old midfielder beat the likes of Harry Kane and Declan Rice to the highly coveted award, to make it the first in his career.
Chelsea came from behind to snatch a late 2-1 win over London rivals, West Ham United, at Stamford Bridge.
“It’s a great three points. The Premier League is tough at the moment,” West Ham manager David Moyes said after the game.
The defending champions laid the marker for the new season with Sunday’s performance.
