Marc Anthony -- I'm Standing By Aroldis Chapman ... For Now

Marc Anthony says he won't turn his back on embattled MLB star Aroldis Chapman -- telling TMZ Sports he's NOT DROPPING Chapman from his new sports management company in the wake of domestic violence allegations.

Anthony's in a tough spot -- because one week after he announced he was launching Magnus Sports and signed Chapman as its 1st client ... reports surfaced that Chapman was being investigated for allegedly attacking his girlfriend.

The incident was enough to scare off the Dodgers -- which reportedly abandoned plans to trade for Chapman because of the allegations -- despite the fact Chapman denied attacking his GF. The Nationals have also said they won't pursue Chapman while his case in pending.

But a rep for Magnus Media tells us bluntly, "He is still a Magnus Sports client."