Alex, who has been out since November and was forced to undergo surgery on a knee injury, trained with the first team after returning from rehabilitating in his native Brazil.
The 28-year-old was badly missed during Chelsea's worst run in the league for almost 15 years at the end of 2010.
On the way back: Alex (centre) has missed a large chunk of the season
He will boost manager Carlo Ancelotti's defensive options, especially in the Champions League, for which new signing David Luiz is ineligible.
'Alex will not be involved tomorrow, but he will be involved on Sunday,' Ancelotti said ahead of Chelsea's last-16 second leg against FC Copenhagen.
That would leave midfielder Yossi Benayoun as Chelsea's only injured player, with the Israel captain having been sidelined by a serious Achilles problem since September.
But the 30-year-old is also expected back soon, with Ancelotti adding: 'Benayoun will come back to train on Thursday or Friday, so this is a fantastic moment for us, to have all the players fit.
'I want to maintain a good motivation for everyone. We need everyone for the run-in.'
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