Frank Lampard has admitted Chelsea's slump in form has coincided with Hakim Ziyech's injury absence.
The Moroccan international aymaker suffered a hamstring problem in Chelsea's 3-1 win over Leeds on 5 December, and has not played since.
The 27-year-old is closing in on a return, but in his absence Chelsea have mustered just one win in six matches in all competitions.
Chelsea have undoubtedly missed Ziyech's defence-breaking abilities, leaving boss Lampard accepting the three-time Ajax player of the year's injury helped halt the Blues' progress.
Asked if Ziyech's injury has played a factor in Chelsea's dip in results, Lampard replied: "I don't think it's a coincidence; I can't rely completely on it.
"You never know the level of how much you are affected even if it seems clear on points because that's not something certain.
"But I think with Hakim, the way he was playing, he was being very effective for us in terms of assists, goal creation, chance creation and some goals.
"And also a real confidence in the way he was playing and we were very fluid at the time.
"He got injured against Leeds when we were in a great run of form, so I think it has been a point.
"You are going to miss players at that level when they drop out and we want him back."
Olivier Giroud headed Chelsea into a 1-0 half-time lead against Aston Villa at Stamford Bridge on Monday night, with the France striker converting Ben Chilwell's cross in fine style.
Anwar El Ghazi's fifth goal in as many games pegged back the Blues however, to leave fifth-placed Villa above Chelsea in sixth in the Premier League table.
Chelsea are scheduled to host Manchester City in west London on Sunday, but that encounter now remains in doubt due to a Covid-19 outbreak in Pep Guardiola's City squad.
City's trip to Everton on Monday night was postponed at the last minute, leaving Chelsea to sweat on whether Guardiola's men can fulfil their weekend fixture.
Lampard insisted safety must remain paramount in the wider societal battle against the coronavirus, while also hoping both that the match goes ahead - and also that Ziyech will be ready to feature.
"Where is he at? There is a possibility Hakim could be fit for Manchester City if it goes ahead on Sunday," said Lampard.
"I will know that later in the week, but he is certainly progressing to the point where he is very close to fitness.
"With Covid I think this is a key time, and a tough time. We have to have safety as paramount.
"From our point of view I will find out in the next day or two because it has to be safe for the two teams, and if not the game won't go ahead.
"I think the Premier League will direct us on that but yes, it is a tough time. I am not surprised because of how Covid is in the UK, it's difficult for everybody.
"Football isn't different all the time, so we will have to see what the Premier League come up with, but safety is paramount."