The Athletic carries a report which explains that the MLS side has suspended the duo for "possible violations of league policy". The club has also said that they have "promptly reported the matter to the League Office, at which point MLS began to gather all available information".
The statement made no mention of the reason behind each player's suspension but the report quotes sources who claim that Fountas was accused of making a racist comment towards Robertha during a recent training session. Following that, Robertha is alleged to have physically assaulted Fountas.
This is the second time that Fountas has been accused of using racial slurs, with Inter Miami making an allegation last season.
MLS found that the allegation was "credible" but they did not take any action, as there was no direct proof that Fountas used such language.
Striker Fountas has been a key player for D.C. United this season, scoring six goals in 17 games, while Robertha has served as something of a deputy, making seven appearances up front and scoring once.
The club face Montreal in the Leagues Cup on Thursday and it is unlikely either man will feature.