Lukaku, who has struggled for form in front of goal this season, missed United's win over Bournemouth on Saturday through injury.
And it appears the Belgium star is struggling in his attempts to regain fitness ahead of an important European clash for United this week.
"I have to wait until Monday," said United manager Jose Mourinho on Saturday after Anthony Martial and Marcus Rashford struck in Lukaku's absence at Bournemouth.
"He needs these couple of days. It was something in training. We don't know, a muscle problem, and we have to wait a little bit."
Right-back Diogo Dalot was also not part of the group that trained on Tuesday morning, with the squad set to fly out to Turin later in the day.
Marouane Fellaini and Antonio Valencia, who have both struggled with injuries so far this season, were both part of the training session at Carrington, as was Paul Pogba, who was in good spirits ahead of his return to the Allianz Stadium.
United No. 9 Lukaku has endured a difficult start to the season, managing just four goals in 14 appearances so far this term.
He was dropped by Mourinho for United's Premier League clash with Everton at the end of October and the weekend trip to Bournemouth, with the Red Devils winning both games in his absence.
The Red Devils’ visit to Turin could be a defining match for Mourinho’s side in Europe this season. A win for Juventus would see them guarantee qualification as group winners, having won all three of their matches in the Champions League so far.
The same result would see United left eight points behind Massimiliano Allegri’s side, with Valencia and Young Boys the other two teams battling for qualification in Group H.
This match will, of course, see Cristiano Ronaldo come up against his former side again, after returning to Old Trafford for Juventus’ 1-0 win over United a fortnight ago.
A late Rashford winner at the Vitality Stadium on Saturday moved United up to seventh position in the Premier League, with Mourinho’s side now unbeaten in the league since the end of September.