Luke Shaw was missing from Manchester United’s training session ahead of their Champions League opener against Sabah on Thursday night, raising concerns that the club’s left-gamble has already backfired.

It’s unclear if the England defender is injured but club sources insist that Wednesday was designated as a rest day for Shaw, suggesting he may be held back for Sunday’s Manchester derby at Old Trafford. Head coach Michael Carrick is expected to address the issue at his pre-match press conference this lunchtime.

United decided against signing cover for Shaw in the transfer window despite showing interest in Newcastle’s Lewis Hall and rejecting an opportunity to sign Rayan Ait-Nouri on loan from Manchester City on deadline day.

The 31-year-old veteran defender started every Premier League game for United last season, but it was widely accepted that he could not go another season without injury. Shaw was replaced by Noussair Mazraoui in added time during Sunday’s 2-2 draw at Everton after setting up a goal for Benjamin Sesko, but it was assumed the change was tactical.

United are relying on a number of back-up options if Shaw is unavailable, with Patrick Dorgu able to play at left back after being used in a more advanced role under Carrick.

Both right-backs, Mazraoui and Diogo Dalot, are capable of playing on the opposite flank, and academy product Harry Amass has stayed at the club when it was anticipated he would leave. New £70million midfield Carlos Baleba – currently out until October – could also be used as an auxiliary left-back.

Carrick spoke about the flexibility of the squad, and Dorgu in particular, at his last press conference on Friday. ‘We obviously had certain things that we needed to fill and try and improve the group. Largely, I think we're in a good place,’ said the United boss.

‘We've got pretty good balance within the whole squad; dynamics, strengths, what we might need in certain games. So, we've got players that are flexible as well throughout the whole team that can play in different positions as and when and if we need it. As you go through a season, you always need that.

‘Patrick can certainly play left-back. It's something he can, we just haven't had to explore that. It's just the way we've been since I've been here. He actually has been injured unfortunately for some time, played three or four times for us last season. So, you could almost argue he's almost like a new player, you know, to have, and it's almost like a new signing in some ways because he's added so much to the group.’