Former Everton boss Martinez knew the player would settle in at United with ease.
“It never crossed my mind that it would be something difficult for Rom," he told Talksport.
"Everyone that plays with him or works with him realises very quickly that he has an elite brain.
“As a footballer, you can have your attributes but the focus he can have is quite unique. When he went to Manchester United he was solely focused on starting well and making a big impact and that is what he has done."