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Alberto Moreno has emerged as a fitness doubt for Liverpool's Europa League showdown with Manchester United.

The Spanish left-back pulled up complaining of pain in his hamstring during Wednesday night's training session at Old Trafford.

Moreno, who had been tasked by Jurgen Klopp with whipping crosses into the penalty box, played no further part in the exercise and was later seen deep in discussions with head physio Chris Morgan.

Liverpool insist it's too early to say whether he will miss the last 16 second leg tie and Moreno will be assessed by medical staff on Thursday morning before a decision is made.

If Moreno misses out then Brad Smith is likely to deputise at left-back.

The Australia international hasn't featured under Klopp since the FA Cup defeat to West Ham last month.

It would be the biggest night of Smith's career. The 21-year-old has only made eight senior appearances and his only previous European outing came away to FC Sion in December.

Jon Flanagan would have been the more experienced option but he isn't registered for Europa League duty.

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