The Belgian, who arrived back at Stamford Bridge in the summer, has missed his side's last five games in all competitions after damaging his ankle in their Champions League victory over Malmo at the end of October.
As revealed by Evening Standard, the forward has now been pictured in training during the international break.
Such an injury saw Lukaku miss out on a call-up to the Red Devils squad, after taking his international record to 101 caps and 68 goals at the beginning of this season.
Following spells with Everton, Manchester United and Inter Milan, the 28-year-old was brought back to London for a fee in the region of £100m in the summer, and he has contributed four goals and an assist in 11 appearances in a blue shirt so far this term.
Thomas Tuchel's side return to action with a trip to Leicester City in the Premier League on Saturday, although it remains to be seen whether Lukaku will be fit to make the squad.