Ninth-placed United's hopes of overtaking three clubs on the last day of the season and completing an astonishing transformation under Blackwell's caretaker stewardship took a blow with James Beattie, Lee Martin and Ugo Ehiogu all rated doubtful for Sunday's season-ending trip to Southampton.

Even victory at the relegation-threatened Saints will not be enough unless other results go the Blades' way – Watford, Crystal Palace, Wolves and Ipswich are ahead of them as five clubs battle for the two remaining play-off places.

Whatever pans out, United chairman Kevin McCabe is understood to be ready to offer Blackwell a full-time contract – talks between the pair are understood to have already taken place.

Blackwell has been a success since taking over from the sacked Bryan Robson in mid-February. When Blackwell returned to the club where he had worked as assistant to Neil Warnock, United were 16th, sliding towards a relegation battle.

Blackwell failed to win any of his first five games but has since turned the Blades into the form team of the division: they have taken 25 points from their last 10 matches.

McCabe, a Brussels-based property developer rated Britain's 397th richest person in the Sunday Times Rich List, watched the 2-1 win over Bristol City that kept his team in with a chance of promotion.

Blackwell, 49, who was previously in charge of Leeds United and Luton, is understood to have also been given the backing of the board.

Given his record, he would appear to be the logical choice to be permanent manager, but one stumbling block is McCabe's future. The chairman, whose personal fortune is estimated at £200m, is looking to step down, although he will retain considerable influence at the club.

Saturday's victory came at a cost, with top-scorer Beattie and central defender Ehiogu both picking up calf injuries in the first half, and winger Martin damaged ligaments.

"I think all three are definitely out for next week," said Blackwell.

The Blades' task is made harder by the fact that the three clubs immediately above them are all at home: Ipswich, level on points with a goal difference better by two goals, host Hull; Wolves, a point ahead but with an inferior goal difference, entertain Plymouth; Crystal Palace, a further point ahead, are at home to Burnley; Watford, three points above United but in freefall and with the same goal-difference, are at Blackpool.