The 23-year-old has not played since suffering the injury in a post-season friendly in Boston in May and Blues boss Frank Lampard conceded he still does not know when the England midfielder will be back in action.
"He's had a few setbacks over the last two months," said Lampard. "He hasn't had a new one, but he's not flying along as much as we would want, and it's not for the want of trying from everybody.
"For Ruben himself of course there's nothing worse than being injured. But at the moment he's no closer to being fit."
One player who is fit again is England striker Tammy Abraham, who headed his 11th Premier League goal of the season then laid on Mason Mount's winner as Chelsea saw off Villa 2-1 at Stamford Bridge on Wednesday night.
Abraham hit 26 goals in 40 matches on loan at Villa last term, and even had the travelling fans singing his name at the final whistle in west London.
The 22-year-old - who sat out the defeat to West Ham at the weekend with injury - boasts a direct hand in a goal in all his 14 league games this term, with his manager suitably impressed.
Lampard hailed Abraham's impact on the other players in the dressing room because of his "enthusiasm and will to win", while claiming his is now showing his full capabilities as a striker.
Chelsea's win ended a run of back-to-back defeats and ensured that they moved six points clear of fifth-placed Wolves, while narrowing the gap on third-placed Manchester City to three points.
The Blues travel to face struggling Everton in their next game at Goodison Park on Saturday lunchtime.