"He trains and doesn't feel anything and then at training the following day he feels something," said Houllier. "Obviously we need to sort that out.
"I played him regularly every game until the Liverpool game and then he got injured and never came back."
Young has been dogged by injury setbacks this season, with the knee issue following hip and ankle troubles, and has been restricted to appearances in only 14 of Villa's 25 league games this season.
Last month Villa signed Tottenham defender Kyle Walker on loan until the end of the season as cover for the England international.
Houllier did not put a time frame on Young's recovery, adding: "Luke saw a specialist yesterday [Wednesday] in London, so it will depend what the results are."
Meanwhile, Houllier is confident winger Stewart Downing will be available for Saturday's home match with Fulham.
Downing suffered an injury during Tuesday's 3-1 loss at Manchester United - a defeat that ended a four-match unbeaten run for the Midlanders.
Houllier said: "Stewart should be okay for Fulham. It was just a kick on the side of his shin."
Source: BBC Sport
Source: BBC Sport