Downing, 31, completed his move from West Ham, for a reported £5.5million which could rise up to £7million, earlier this month and Adomah claims his arrival has given the club a major boost.
“Stewart is a Premier League player and could’ve stayed there,” Adomah told Boro’s official website.
“But he’s come back to his hometown club and the supporters will appreciate him and hopefully he can do well for the team.
“That’s the mentality of someone like Stewart Downing – he’s coming back to his hometown club to help us out to achieve success and get back to the Premier League.
“Having a quality player like that will only attract more quality players, which will help Boro in the future and hopefully in this, or the next, transfer window.
“For someone like him to come here knowing he could’ve stayed in the Premier League will attract more players to the club.
“For the chairman to invest in a player like that, hopefully it will be a success.”