An unnamed Championship side submitted the offer on Thursday, bringing McCormack’s future back into the spotlight with the start of pre-season training just two weeks away.
The bid materialised after two days of speculation linking Derby County – last season’s losing play-off finalists – and recently-relegated Norwich City with the 27-year-old Scotland international.
United have not commented on the bid and the club behind it are yet to be identified but a £5m proposal is likely to signal the start of a concerted effort by several teams to sign McCormack.
Leeds are still considering Thursday’s offer but are expected to reject it, with Cellino understood to value McCormack at closer to £7m.
The Italian, who bought United at the start of April, is reluctant to lose the striker after his 29 goals for the club last season but he is also unwilling to raise McCormack’s salary further, less than 12 months after the captain signed a new four-year contract.
Premier League interest in the ex-Rangers trainee became apparent in January when West Ham and Cardiff City bid for him, and Newcastle United made an enquiry about McCormack at the beginning of last week.
Cellino claimed to have turned down a formal offer from St James’ Park, a claim which Newcastle moved quickly to deny, but the 57-year-old businessman spoke again of his desire to keep McCormack at Leeds, saying: “Newcastle made an offer for McCormack and I said ‘no’.
“He is happy to stay here and that made me proud.”
Despite their approach in January, Cardiff are not behind the latest bid for McCormack. Norwich, however, hold the financial advantage of parachute payments after dropping out of the Premier League in May and have routinely looked to Elland Road for signings in the past three years.
Derby, meanwhile, fought their way to the brink of promotion last season, beaten by a stoppage-time goal from QPR’s Bobby Zamora during the play-off final at Wembley. The Midlands club, however, appear to be distancing themselves from this week’s approach.
Cellino is due back in Leeds tomorrow, having spent this week in Italy finalising the sale of Serie A club Cagliari, the team he bought in 1992.
McCormack’s future will be high on his agenda, alongside the appointment of a new head coach after a week of few developments on the managerial front.
Reading academy chief Eamonn Dolan remains the leading candidate for the post, though Cellino would be forced to pay a six-figure fee in compensation for him, and former Forest Green Rovers manager Dave Hockaday is also in contention.
Meanwhile, United’s supporters have until the close of business this evening to buy season tickets for next term at frozen renewal prices.
The club’s ticket office at Elland Road will be open until 5.15pm to accommodate last-minute applications on top of the thousands that have already been submitted.
It is also the last chance for supporters to guarantee their existing seats.