The 18-year-old came on for the final 12 minutes of the game and showed flashes of brilliance during his cameo.
He went past two Croatia defenders with five minutes left on the clock, before delivering a cross that was nearly turned in.
And Dier said he was excited to see Sancho in England colours again.
“Jadon Sancho was fantastic when he came on, very direct and confident, fast and technicality very gifted,” Dier, 24, said.
“Hopefully he can be a big part of England in the future, he needs to keep working hard.”
Dier went close to giving Gareth Southgate’s men what would have been a deserved lead in the first half, but his header came back off the post.
Tottenham team-mate Harry Kane also saw a header come back off the crossbar, while Manchester United’s Marcus Rashford should have done better when one-on-one with Dominik Lovakovic.
A strange game was played behind closed doors because of a UEFA sanction against Croatia.
While some England fans still travelled and managed to watch the game from a hill behind the stadium, Dier admitted the ban affected the atmosphere.
He added: “It was strange, the first experience of it for all of us. A couple of fans got up on the hill but once the game started it was in the back of our minds.
“We could easily hear the ones on the hill, they were the only ones here.”
Southgate opted to go with a back four despite playing three in defence through the successful World Cup campaign in Russia.
But Dier was pleased with the system, adding: “This is a formation that suits me well and works towards my characteristics. The team did well and important to have the flexibility of different formations.
“I will always give my best when I play, no matter what the formation or team selection.
“I thought I had scored and when I looked back I felt it was going in. It was unlucky and disappointing.”