The Wales star has impressed since making the £15million switch to Liverpool from Swansea in the summer.
But the chasing from Modric in Tuesday's 2-0 World Cup qualifying defeat in Croatia made him realise he still has a long way to go.
Allen said: "Modric is some player.
"I think he showed me that I have still probably got quite a lot of work to do in order to get anywhere near his level.
"He's a top player but for me personally it was disappointing because he controlled the game for them and was comfortable throughout."