United go into their quarter-final second-leg matchup against the La Liga side at Old Trafford having built up a 2-0 lead from the first leg in Spain last week.
But Norwegian boss Solskjaer will be without Harry Maguire, Luke Shaw and Scott McTominay, who are all serving one-match suspensions.
Eric Bailly is also set to miss out due to a positive coronavirus test while on Ivory Coast duty which has left Solskjaer with just two senior centre-backs: Victor Lindelof and Axel Tuanzebe.
With extra cover at centre-back required, 18-year-old Fish – a regular for the Under-23s this season – has made United’s senior squad for the first time, and Solskaer has plenty of faith that the teenager could step up to the plate if needed.
He’s a very good talent, a composed centre-back with physical stature,’ Solskjaer told the club’s official website.
He’s one we believe in.
Of course, we’ve lost a few defenders so to have Will on the bench is a good experience for him and he’s there if he’s needed and he’s ready.
That’s why we have a good, strong squad and that’s why these players come in every day knowing that at one part of the season, you’ll have to contribute and make an important contribution.
‘So I trust the ones that come in.‘
Further forward, Anthony Martial remains sidelined through injury and Marcus Rashford, who scored the opener in last Thursday’s first leg, is also an injury doubt after missing training before the clash at Old Trafford.
‘Marcus didn’t train on Wednesday so I’ll have to give him as much time as possible, his foot is still a little bit sore, but, fingers crossed, he is available,’ Solskjaer added.
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