According to The Daily Mail, Rangers had to pay Peterborough a transfer fee of £2.8 million for the services of the 24-year-old striker.
Hall, who worked with the Northern Ireland international striker at St Ives Town, believes that Washington will show his quality next season.
“It took him a while at St Ives to get going, but once he did he was pretty much unstoppable,” Hall told Get West London about Washington. “If you look at his Newport and Peterborough records, he was quiet there for the first few months.
“But once he feels comfortable, settled in, he does what he’s good at, and I'm sure that will be the same at QPR.”
Washington failed to score a single goal in 15 Championship appearances for QPR during the second half of the 2015-16 campaign.
The striker, though, did create nine chances, and had a pass accuracy of 65% and a shot accuracy of 57%. The Northern Ireland international also won eight headers, four dribbles and three tackles.
It is not easy to make the step-up from League One to the Championship, but QPR fans should be optimistic about Washington scoring a lot of goals for the team next season.