Holders Manchester City will again be without John Stones, Jack Grealish and Mateo Kovacic for their Champions League opener against Red Star Belgrade on Tuesday.
Defender Stones has been out since the Community Shield, Grealish suffered a knock prior to the recent international break and Kovacic suffered a problem while away with Croatia this month.
The trio are close to returning, but the visit of the Serbian side comes too soon.
Manager Pep Guardiola said at a press conference: "They are getting better, but not for tomorrow. Hopefully for Nottingham (Forest) or the next games."
City finally won the Champions League last season after years of near-misses and Guardiola feels his team may find it easier to defend their crown than win it for the first time.
"The hardest thing is to win the first one," Guardiola said.
Meanwhile, Kyle Walker has confirmed he has been voted as one of City's five 'captains’ for the coming season.
The England right-back has worn the armband so far this season following the departure of Ilkay Gundogan.
The squad have held a vote in recent days to determine the German’s permanent successor.
Walker said: "The vote’s come in and the five captains chosen. We’ll lead the team for the season.
"The order doesn’t matter. It’s down to me, Kevin (De Bruyne), Rodri, Ruben (Dias) and Bernardo Silva."
Nat Phillips trained with the Celtic squad ahead of their opener against Feyenoord on Tuesday.
The on-loan Liverpool defender saw his debut cut short on Saturday when he rolled his ankle towards the end of the first half against Dundee.
Phillips was replaced by Gustaf Lagerbielke at half-time, although he was only scheduled to play an hour in his first appearance since his loan move.
Liam Scales is Brendan Rodgers' other recognised centre-back option with Cameron Carter-Vickers, Stephen Welsh and Maik Nawrocki missing through injury.
Scales has started each of the last three matches and helped Celtic keep clean sheets against St Johnstone, Rangers and Dundee.
Rodgers’ squad trained at Celtic Park before flying out to the Netherlands on Monday afternoon ahead of Tuesday’s clash at the De Kuip Stadium in Rotterdam.
Follow a live blog of Celtic versus Feyenoord on RTÉ.ie/Sport and the RTÉ News app. Watch the Champions League encounter from 7:30pm live on RTÉ2 and RTÉ Player
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