Alli has been named in Roy Hodgson's England ranks to face Estonia and Lithunia in the coming week, after playing just seven Premier League matches in his career to date.
The 19-year-old has impressed whenever he has taken to the field, but there have been suggestions that he has been rewarded with an England call-up far too early.
However, Merson believes that now is the perfect time for Alli to prove he is ready to make the step-up to international level, and he predicts that former Milton Keynes Dons man can play a prominent role for the Three Lions for the next 10 years.
"I'm not surprised," the former Arsenal player said. "It's a great time to put him in the squad, after we've qualified. Before the end of the season there will be a couple of players who miss out, or who get injured. You can't then rely on someone to come into a squad who has never played before.
"I think it's a great opportunity for the lad. He's going to play for England for the next ten years, in my opinion, he's a real, real good player.
"We can't wait until they're 24 or 25, we have to get them in now. These are proper games, they're not friendlies, and if he's going to be in there for the next 10 years, why not put him in now?"
Alli has been a revelation for Spurs so far this season, with his form surpassing many people's expectations.
Alli was purchased by Spurs as one for the future, but after an impressive pre-season he has been thrown straight in at the deep end.
The powerful midfielder has formed an excellent partnership alongside Eric Dier in the heart of Spurs' team, and he looks set to be vital for his club and country in the present and future.