De Gea struggled on the pitch in his first season in England following a £17.8 million move from Atletico Madrid in the summer of 2011, with inconsistent performances punctuated by high-profile mistakes.
And Steele has now revealed the Spaniard's difficulties were the result of a number of issues, including weight, diet and attitude.
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"We pushed protein drinks on him straight after training. We physically made him drink. We had him in the gym a lot. He hated it. They don't do the gym in Spain as much. We needed to build his core strength.
"One issue with him was that he was just 71 kilos. We worked with him on and off the field to make him more powerful. We changed his lifestyle.
"He would finish training and want to go home. When I told him to come back in the afternoon he'd ask: 'Why?'
"[He could] be very solemn in training, he was always better in a group. Solemn because he was tired, mentally and physically. I told him he needed to train better, that he was on show every day.
"There were times in his first season when he trained poorly. I told him that players made decisions for managers, not the other way around. That he should be first, not last out for training."
Steele also admitted he was forced to learn Spanish to communicate with De Gea, thanks to the United No.1's unwillingness to apply himself in his English language classes.
"We needed to be able to communicate with each other straight away, even if it was just the basic terms," he added.
"David is lazy in his desire to learn English. So I learned. I kept telling him to work on his English."
When asked whether De Gea - who has been linked with a return to Spain to replace the outgoing Victor Valdes at Barcelona - is settled at Old Trafford, Steele replied: "Yes, but there are still issues.
"He's living in a huge house with his mum and dad. He's got a beautiful girlfriend who lives in Barcelona. We pushed him to move on his own. Living with your parents helps and hinders."