Forced to miss most of last season having undergone knee surgery in September, O’Connell appeared on course to be declared available for selection this summer before suffering a set-back in his battle to recover from knee surgery.
With manager Slavisa Jokanovic now being informed the 27-year-old is set to miss the entirety of United’s warm-up programme ahead of his first campaign in charge, the Serb has held a series of meetings to discuss the situation - and how it affects his recruitment plans - at the club’s training camp in Spain.
Given the uncertainty surrounding Kean Bryan’s future at Bramall Lane - he has yet to reveal whether he plans to sign the contract proposal presented to his agent last month - Jokanovic has now given defence the same priority status as midfield during the transfer window.
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Brighton and Hove Albion’s Matt Clarke, who spent last term on loan with Derby County, has been linked with a move to South Yorkshire although Jokanovic could also use the loan market to source a replacement for O’Connell.
Previously of Brenford and a pivotal figure in the 3-5-3 system which saw United win two promotions under Jokanovic’s predecessor Chris Wilder, O’Connell’s absence is regarded a major factor behind their relegation from the Premier League last season.
Having seen John Lundstram join Scottish Premiership champions Rangers and given Arsenal’s interest in Sander Berge, bolstering his options in the centre of the pitch was initially the most pressing item on Jokanovic’s ‘to-do’ list.
But the emphasis has now changed, with strengthening the heart of United’s rearguard now given equal billing.
Meanwhile, United’s home game against Bristol City, which had been scheduled to take place on November 27, has been rescheduled for November 28 at the request of the police.
United have also unveiled details of their warm-up schedule ahead of August’s return to competitive action, with matches against Europa Point of Gibraltar, Doncaster Rovers and Norwich City all set to be streamed live.