Black, 33, has yet to begin training with the main playing group and is in extreme doubt to be fit in time for the Lions' campaign opener against the Western Bulldogs at Etihad Stadium on March 30.
Voss said the three-time premiership hero and Brownlow medallist was pushing hard for a recovery from the knee injury, which has taken much longer than expected.
"We're hoping to get him on the track very very soon, but we're now three weeks out and he's not training with the main group," Voss admitted.
"It's going to be pretty tough for him to be able to get up."
Black underwent surgery on his knee way back in December and had been expected to have resumed full training close to the new season.
Voss said while he hasn't given up on his premiership mate, Black's chances of Round 1 action were getting slimmer by the day.
"He's an outside chance but he's pushing it a little bit now," Voss said.
"This week we'll have a look at Blacky and what his timeline looks like, but it's starting to drift a little bit."
Black, at 33, is the oldest member of the Lions playing group and defied speculation at the end of last year that an injury-plagued 2012 season was set to be his last.
Despite constant injury battles, Black still managed to average 23 touches over his 18 appearances last season.