Delap, 22, has bagged 10 Premier League goals for a newly-promoted Blues side struggling towards the bottom end of the table.
He's the third highest-scoring Englishman in the league, but hasn't been picked for a squad that includes the forward trio of Harry Kane, Dominic Solanke and Marcus Rashford.
"Look, it's a really good achievement to be in England's Under-21s," said McKenna. "There's a European Championship in the summer, which is a really big tournament. I know, from an England development pathway, they increasingly put a lot of stock in those tournaments.
"Not to pick Lee Carlsey's squad, I'm sure he'll have his own opinions, but I'm pretty confident that we'll have three of the England forwards in that squad (Delap, Omari Hutchinson and Jaden Philogene) which I think is a great thing for the football club and a great sign for the future."
He added: "Look, I think there are many different ways of looking at it. I don't think it's always clear cut. I've not been an international manager or a technical director, though I know John McDermott well.
"Was there any big surprise from our point of view? Honestly, we haven't put too much stock in it ourselves and I know Liam's the same, he's just really, really focused on his job this season and doing the best he can.
"He's a young striker, as we've said many times, who's playing his first season of Premier League football and, in some ways, playing his first season as a leading No.9 for a team.
"He's made fantastic progress, put in some outstanding performances and there's still a lot to come.
"With what he's done for a newly-promoted team, you could certainly make a strong case that merits being in the top band of players in the country.
"On the other hand, I know that there will be a lot of other things that come into it. I'm pretty sure, without speaking for those at The FA, that they've looked at this major Under-21s tournament, taking that seriously and giving their best young players the chance to play in them because they believe that can be productive for them in playing in senior tournaments further down the line, which I believe Liam will do.
"I'm pretty sure there's a good logical argument why a player who's stepped up so well is better served remaining with the Under-21s.
"He's certainly been one of the best centre forwards in the Premier League, but on a big picture scale there are lots of reasons to understand why players get placed in different squads."