As prior of the convent San Felipe Real Madrid, he was presented by the king of Spain to the bishopric of Puerto Rico on March 28, 1576.
Diego made pastoral visits throughout the island and undertook the task of reform as laid down by the Council of Trent.
The people were living very poorly, The diocese had very few priests and religious to meet the spiritual needs of parishioners.
There are real problems with officials, who accused him of being a smuggler, taking advantage of his position as bishop.
Additional health problems and other personnel asked the king to permit him to travel to Rome and expose it to the pope.