Keenan will return to Ireland after fracturing his thumb in training at a pre-tournament training camp in Portugal, the Irish Rugby Football Union (IRFU) said after confirming that Aki did not travel to the camp.
The decision to replace the 35-year-old with uncapped Jude Postlethwaite followed a misconduct complaint arising from Aki's club Connacht's 34-23 defeat by Leinster in the United Rugby Championship (URC) on Saturday.
Aki is alleged to have engaged with match officials on several occasions in a manner which may be deemed to be in breach of the league's disciplinary rules related to misconduct, the IRFU said.
"The IRFU does not tolerate any form of disrespect shown towards match officials and does not condone actions that fall below the standards expected of players representing Irish rugby," it said in a statement.
The IRFU did not give a timeline for Keenan's return from injury. The British and Irish Lions fullback has not played this season following a hip issue.
Keenan joins an injury list of almost a dozen players, including centre Robbie Henshaw, flanker Ryan Baird, winger Mack Hansen and all three of Ireland's first-choice loosehead props.
Andy Farrell's side start the Six Nations against defending champions France in Paris on February 5.