Depay arrived at Old Trafford in the summer of 2015 from PSV Eindhoven but has failed to live up to his £25million price-tag.
He struggled for form last season and managed just seven goals in 48 appearances under former manager Louis van Gaal.
The 22-year-old has made just one start since Jose Mourinho took charge at United in the summer.
But Martins Indi is confident his Dutch compatriot will come good and told the Mirror: "I know his potential. You know his potential. Everyone knows his qualities."
The Stoke defender, who joined on loan from Porto in August, also believes his former Holland boss Louis van Gaal was a success during his two years at Old Trafford.
Martins Indi added: “Van Gaal brought success. He won the FA Cup. That’s success, isn’t it?
“Of course, a club like Manchester United needs success. They can’t have a long time without it. But winning the FA Cup was just that.
“The other most important thing in __football is to give opportunities to young players – the next generation – and he did that too.
“He did it with me. I was 22 at the World Cup and to play seven games showed a lot of faith in me and I am grateful for that.
"In my opinion, he has done the same also with Marcus Rashford and Timothy Fosu-Mensah.
“As a manager, the only other thing you can do is create a legacy. And I think Van Gaal did that too.”