Newcastle United had been close to securing a deal for Alli, then at League One side MK Dons, in January.
But Tottenham won the race to sign him before loaning the midfielder back to the club, which won promotion to the Championship.
Alli has quickly established himself in Mauricio Pochettino's side and won two senior England caps this season.
United head coach McClaren watched Alli while he was in charge at former club Derby County.
"I watched him when I was at Derby and he was at MK Dons, and we talked about trying to get him," he said. "He's really surprised everybody, and then he made his debut for England wow! He's a very good player."
Spurs paid £5million for 19-year-old Alli, and McClaren believes his development since proves that there is talent in the lower divisions.
He said: "Without doubt. That's one of the things I enjoyed about my time at Derby, seeing all the young players coming through and the loan players.
"It is like your Charlie Austins and your (Jamie) Vardys there are still people down there, that if they keep working and put the effort in, they can make it. Even in the Championship, players are going up the likes of Callum Wilson, Troy Deeney, (Odion) Ighalo.
"There are some good players."