In news first reported by TSN 690's Joey Alfieri, Piatti is expected to miss at least three weeks with an injury.
Garde confirmed the news Friday at his final availability prior to Saturday's match against FC Dallas at Stade Saputo (7:30 pm ET | TVAS, MLS LIVE on ESPN+ in the US), but clarified the injury was to his adductor, not his groin.
Piatti has played in only eight of the Impact's 26 league matches so far this season after an earlier knee injury kept him sidelined for nearly four months altogether.
He immediately contributed on his return, scoring the deciding goal in Montreal's 3-2, two-leg win over York9 in the third qualifying round of the Canadian Championship.
He went on to play in three MLS matches, but was not available in the second leg of Montreal's semifinal at Calgary this past Wednesday, a 1-0 Impact win.
Piatti has three goals and an assist over his injury-shortened 2019 MLS campaign after hitting the net at least 16 times in each of his previous three MLS seasons.