Sir Alex Ferguson believes a rift between Jose Mourinho and Andriy Shevchenko is 'at the core' of Chelsea's current problems.
Ferguson claims Mourinho was 'happy enough' when the pair had dinner in November and cites reports of a rift with record signing Andriy Shevchenko as the catalyst for the current speculation.
He said: 'I think that whatever has happened, the situation with Shevchenko seems to be at the core of it.'
The Premiership champions are enduring their most difficult season under Mourinho, and are six points adrift of Ferguson's United at the top of the table.
Rumours abound over whether the Blues' Portuguese boss will quit before the start of the next campaign amid reports all is not well in the Chelsea camp.
Ferguson compared the effect of such speculation to what was meant to be his final season at Old Trafford in 2001-02, when United finished outside the top two for the first time in the Premiership.
He believes the speculation over Mourinho's future could unsettle Chelsea in the same way Manchester United were affected by his own decision to retire.
He said: 'I announced it at the start of the season and I just got a feeling as the season progressed a lot of them were saying 'ah well, he's not going to be here any longer...' and maybe some genuine concerns about 'who's going to take his place?' and just a relaxation possibly.
'But there was no doubt it had an effect and, once I decided not to retire, there was a genuine pick up again... it's amazing.'