The Championship side will no doubt rue the loss of an exceptional player, while the man messaging him on Twitter is also out injured, meaning an existing blow for Dougie Freedman's side.
But West Ham clearly produced an offer too good to turn down - as Forest have had no trouble turning down offers for their star man in the past.
Assombalonga, meanwhile, seemed gutted that his room-mate has gone, although he he was jovial on social media, wishing the Hammers winger the best of luck and warning him that he'll be watching!
So my room buddy @Michailantonio is gone good luck to you my brother, I'll be watching— Britt Assombalonga (@BrittOfficials) September 1, 2015
Nottingham Forest's early-season struggles are unlikely to be helped by the knowledge they won't be able to call upon Antonio in the future. West Ham, however, know they will have a very good wide man on their hands.
Bakary Sako has made a seamless transition from the Championship to the Premier League and the Forest man was not too far apart from him statistically in the second tier last season.
He'll certainly be missed at the City Ground - but West Ham supporters won't mind.