The Scotland defender appeared to suffer a knee injury early in the second half of Motherwell's final warm-up for their third qualifying round tie against Kuban Krasnodar.
McManus walked off with a slight limp before being replaced by Fraser Kerr.
Fellow centre-back Shaun Hutchinson also went off with 12 minutes left after appearing to be hit by cramp.
Motherwell, who host the Russians on Thursday, started slowly and fell behind in the 12th minute when Matt Derbyshire swept home from six yards following a cross from the right by Dexter Blackstock.
The hosts improved and came close through Bob McHugh's long-range strike and John Sutton's well-saved header before losing McManus.
James McFadden started after being withdrawn at Cowdenbeath on Saturday with a tight hamstring but was not given his first full game of pre-season, going off somewhat reluctantly in the 63rd minute.
Lee Hollis made good saves from Andy Reid and Adlene Guedioura while McHugh forced a good stop from former Dunfermline goalkeeper Dorus de Vries before equalising with eight minutes left. The striker fired into the roof of the net from 12 yards after good work from substitute Zaine Francis-Angol.