The Manchester United forward has returned to the England set-up after being named in Boothroyd's squad for the Euro 2021 group phase.
It marks his comeback after Greenwood was sent home from the seniors, after breaching coronavirus regulations with Phil Foden, following his debut in a 1-0 win in Iceland in September last year.
The Young Lions face Switzerland in their opening Group D game in Koper, Slovenia, on March 25, with the group stage being held this month due to the coronavirus pandemic.
Boothroyd said: "We treat every player as an individual and we think very carefully when and where we pick them. Clearly in Mason's situation, it was right that he wasn't right to play for his country and to stay where he was.
"That period of time has done him the world of good, he's getting more minutes there. That's a positive thing. We hope he can continue that with us.
"He's got a lot of support around him from his club and from us. Gareth (Southgate) is speaking regularly to Ole (Gunnar Solskjaer) in this case and we think he's a top player. Has he got something to prove?
"Every time he gets on the pitch there is an expectation of him because he's a very good young player. But he is young, he needs guidance and help. Gareth will speak to him before we meet up. I'll speak to him to see how he is and see how we can get the most out of him.
"We're talking about a player here who is nowhere near his potential. But he's growing up, he's learning about himself as a footballer and a person. We're really looking forward to seeing what he can do on the big stage."
Greenwood is joined by Chelsea's Callum Hudson-Odoi while uncapped Josh Griffiths, Emile Smith Rowe and Noni Madueke of PSV Eindhoven have earned their first call-ups, along with Tottenham's Japhet Tanganga.
Despite his lack of action at Arsenal, Eddie Nketiah – who top scored in the qualifiers with 13 goals to become the Under-21s' record scorer – is selected along with Everton duo Ben Godfrey and Tom Davies, while Aaron Ramsdale is expected to be Aidy Boothroyd's number one.
England also face Portugal in Ljubljana on March 28 and Croatia in Koper three days later, with the knock-out stages in May and June after the tournament was split up.
Boothroyd is also unable to select Bayern Munich's Jamal Musiala after the forward chose to play for Germany.
The 18-year-old, who was born in Stuttgart, made two appearances during the Under-21s' qualifying campaign, scoring on his debut against Albania in November, but pledged his international future to Germany last month.
"Jamal is a very measured young man. He's obviously thought it through," Boothroyd added. "When you're working in Germany, speak German and you live with your German mum, it's very difficult to persuade him that he's actually English.
"It was his decision, he talked about having a heart for Germany and I totally understand that. We wish him the best of luck and we move on and focus on the players who want to be part of what we want to build."