Chelsea midfielder Lesley Ugochukwu is expected to be out for up to six weeks, handing returning loanee Andrey Santos an increased chance to stay in the squad.
Ugochukwu, 19, re-injured his hamstring on Christmas Eve away at Wolves having been sidelined for a month before the trip to Molineux.
The France Under-21 international has been an important squad player for Mauricio Pochettino so far this term, racking up 13 appearances across all competitions, but is now set for another spell on the sidelines.
Chelsea have recalled Santos, another 19-year-old midfielder, from a disappointing loan spell at Nottingham Forest, where he made just two appearances in the first half of the season.
The one-time Brazil international is back training in Pochettino's first-team squad before Preston visit Stamford Bridge in the FA Cup third round on Saturday.
Santos is not rushing to find another loan despite interest from Crystal Palace and several clubs in the Championship, with further injuries affecting Chelsea's midfield.
Their plight could give the highly-rated defensive midfielder, who is often compared to Manchester United's Casemiro, a chance to make an unlikely debut in January.
Romeo Lavia and Carney Chukwuemeka are both out injured, while Enzo Fernandez is struggling to play full matches due to a painful hernia problem.
Chelsea are not expected to be very active in the January transfer window but are currently focusing any resources they have on signing a striker.
Players must be sold if Chelsea are to spend significant sums due to restrictive Financial Fair Play (FFP) regulations. Ian Maatsen is attracting interest from Borussia Dortmund and Bayern Munich are monitoring Trevoh Chalobah's recovery from a long-term hamstring issue.
Conor Gallagher, one of only two fully fit senior Chelsea midfielders alongside Moises Caicedo, has been linked with a move to Tottenham but is unlikely to depart in January despite having only 18 months left on his contract.
