A lawyer for the former Cleveland Browns quarterback told the Associated Press news agency that prosecutors had told him a charge was coming.
A Texas grand jury has agreed a misdemeanour assault charge for family violence, the lawyer said.
Colleen Crowley says he confronted her at a Dallas hotel and hit her as they drove back to Fort Worth.
The Class A misdemeanour carries up to one year in jail and a $4,000 fine.
Lawyer Robert Hinton told AP the charge was expected on Tuesday and he expected a judge to set bail.
Despite success playing football as a university student, Manziel - nicknamed "Johnny Football" - has struggled with substance abuse and personal issues in the professional league.
Manziel was released from Cleveland Browns in March.
Troubled NFL quarterback Johnny Manziel is expected to be charged over an alleged assault on his ex-girlfriend.