The out of contract player will turn out for the reserves before attempted to break into the first team.
Chelsea were forced into action when Michael Essien suffered a serious anterior cruciate injury.
This left just Mikel to fill the defensive midfield role vacated by Claude Makelele departure from SW6 in the summer.
The 33-year-old was released by German side Hertha Berlin. He will wear the number 27 shirt.
Mighty Port Vale have returned so-called wonderkid Tom Taiwo after a month's loan.
Chelsea snapped up the 18-year-old from Leeds United.
Following their 1-1 draw at home to Manchester United, Chelsea progressed to the fourth round of the Carling Cup yesterday with a 4-0 rout of Portsmouth.
They travel to Stoke City this weekend.