Phil Jones, Chris Smalling and Tom Cleverley all accompanied Rooney to England's training base in Watford ahead of Friday's qualifier with Bulgaria in Sofia, and striker Danny Welbeck would almost certainly have joined them had it not been for the hamstring injury he sustained against Arsenal at the weekend.
"They could be key players at Euro 2012," said Rooney.
"They have done well for the Under-21s in the tournament just gone.
"The manager (Capello) has recognised that and brought them into the squad. We didn't do much in training today but you could tell they were comfortable around the other players.
"They know a few players here and have settled in quickly. Hopefully they will get a chance and do well."
All four United players, including Welbeck, were part of Stuart Pearce's squad at this summer's Under-21 Championships.
Jones and Smalling were two of only a handful to emerge with any great credit from a largely disappointing campaign.
Yet Smalling did so at centre-half. Now he is scrapping it out with Micah Richards for the right-back role on Friday, after impressing Capello with his performances for United out wide.
"He has done brilliant for us at right-back this season," echoed Rooney. "Phil Jones can play at right-back too, so the two of them can play in different positions. During a tournament it is so vital for the manager to have that."
Source: PA
Source: PA