The Red Devils are confident that a transfer fee of £9 million (€10m/$11m) will be accepted by Juventus, who are desperate to sell the Croatian veteran.
Mandzukic is expected to sign an 18-month contract until the summer of 2021 and would earn a salary of £4.4m (€5m/$5.5m) a year.
The Red Devils have failed to score more than one goal in a game since they beat Chelsea 4-0 on the opening weekend of the season.
They currently languish in 12th place in the Premier League and are in the market for a short-term option in attack to help propel them up the table.
Mandzukic has enjoyed four excellent years at Juventus since arriving from Atletico Madrid in 2015, winning four straight Italian league titles and scoring 44 goals in 162 appearances.
However, the 33-year-old is now surplus to requirements in Turin, with new coach Maurizio Sarri deeming the giant striker unsuitable for his style of football.
Juventus spent the second half of the summer trying to offload Mandzukic and offered him to United during the latter part of the window.
There is interest in his services from China and the United States, but United are confident of completing a deal when the transfer window reopens in January.
Mandzukic would be expected to lead the line for United and take the pressure off Marcus Rashford, who has struggled to find the net following injuries to Anthony Martial and Paul Pogba.
Under-fire United manager Ole Gunnar Solskjaer retains the support of the board at Old Trafford and only a disastrous run of form would shake them from the belief that the Norwegian is the right man to take the team forward.
The club’s hierarchy will continue to back Solskjaer in the market. Mandzukic is not part of a long-term strategy but the club recognise that the team are short of options and goals in attack.
Solskjaer and CEO Ed Woodward sanctioned the sale of Romelu Lukaku this summer without signing a direct replacement.
United briefly considered a free transfer move for Fernando Llorente but he ultimately joined Napoli, leaving Mandzukic as the No.1 target, and a January deal appears likely.