The former West Bromwich Albion midfielder had one of the best seasons of his career at Kilmarnock last season and became a fan favourite at Rugby Park.
Kilmarnock fans would have loved to have been able to welcome Mulumbu back to their squad this season, but the midfielder was keen to explore his options.
He may well have a better one with Celtic, who can offer him the chance to play Europa League football and a strong chance of winning a title medal.
What he will not get is a guaranteed starting place, which he earned at Kilmarnock.
Competition for places at Celtic means Mulumbu will have to work his way up from a role as a back up player.
Celtic fans have had a mixed reaction to his imminent addition, and it remains to be seen if he can turn them around.
Kilmarnock supporters would have been 100 per cent in favour of his addition.
Now their views are more mixed. Here is a look at how they reacted to his Celtic switch...
mulumbu would be worshipped at killie.. probs just aner squad player at celtic— Jokey (@jokeymorrisjnr) August 30, 2018
With the news that Mulumbu is very probably away to Celtic, it's made me very sad. Not only because I love him, because so many Celtic fans are completely underwhelmed by the signing. You guys have no idea how good you've got it, absolutely no clue at all.— Andy K (@ShandyK_) August 30, 2018
@ClydeSSB— Scott Campbell (@Mr_SJCampbell) August 30, 2018
Gutted YM isn’t back @KilmarnockFC
Celtic fans are nowadays as expectant & arrogant as Old Gers’ fans of the 90s!
Mulumbu bossed Celtic & embarrassed Gers lst yr.
If Gers thought they’d win lge this year,think again,YM signing wraps up the lge.
Celtic,Enjoy his magic.
He's dead to me now!— Robert Dykes (@boabdykes) August 30, 2018
Wonder if Mulumbu will do a Mo Johnson and come back to Killie— Ethan Taylor (@EthanTee99) August 30, 2018