The Blades will “firm up” their interest with a move for Swansea City striker Oli McBurnie looking increasingly unlikely.
McBurnie is on United’s list of targets, but could now link up with former Swans boss Graham Potter at Brighton and Hove Albion.
Assombalonga is said to be Chris Wilder’s preferred alternative to the Scotland international. Brentford’s Neal Maupay is also on his shortlist.
Boro are ready to cash in on Assombalonga if they receive close to the £15m they paid Nottingham Forest for the forward in 2017. The club are keen to cut their costs and the 26-year-old is among their highest earners.
However, the Sheffield Star claims there could be an issue in any potential deal for Assombalonga. That is because the attacker is due to take part in next month’s African Cup of Nations.
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Assombalonga is in the Democratic Republic of Congo’s provisional squad for the tournament – which starts on June 21. Negotiating a move while he is taking part could prove complicated.
Depending on how far his country progresses, Assombalonga could be away until the final on July 19. That is three days after the Blades play their second pre-season friendly. He would also miss their training camp in Portugal.
The former Forest man became Middlesbrough’s record signing when he joined them in the summer of 2017. He scored 30 goals in 69 games during three injury-ravaged years at the City Ground.
Prior to that Assombalonga had also been prolific at Peterborough United, Southend United and Braintree Town.
In his two years at the Riverside he has 31 strikes in 93 appearances. This season Assombalonga scored 16 times in all competitions.
It could be argued that Tony Pulis’ defensive style limited the striker. Despite finishing seventh, the North East club were the fourth lowest scorers in the Championship in 2018/19.