Earnshaw used his right foot to deflect a Jonathan Osorio shot into the net in the 16th minute, supplying all the offense TFC (6-17-11) would need behind three saves from Joe Bendik.
Troy Perkins made six stops for Montreal (14-13-7), which is seeking its first-ever MLS playoff berth.
The Impact will need either New England or Houston to lose in their Sunday finales, or for Chicago to fall while scoring four or fewer goals in its final game.
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