Rafa Benitez has become "frustrated" at a lack of progress in the transfer market, according to reports.
The club has signed just one player – winger Christian Atsu – so far this summer.
And The Mirror claim that Benitez is "unhappy after missing out on a string of targets".
The Daily Mail report that Benitez's "future as manager is uncertain after a series of transfer setbacks".
Benitez had wanted the club to do business early this summer ahead of the club's return to the Premier League.
The club has already lost out on a number of targets, including Chelsea striker Tammy Abraham, who is set to join Swansea City on a season-long loan.
Newcastle, however, could finally sign Eibar defender Florian Lejeune next week for £8.8million.
United owner Mike Ashley pledged to back Benitez with "every penny" that the club generated through promotion last month.
And the slow progress in the transfer market is set against discussions with potential investors.
Benitez's players report back for the start of pre-season training on July 3.