The Spain U21 international moves to Everton in a permanent transfer.
"As you know, since Jan Mucha has moved on, Tim Howard was the only goalkeeper at the football club," Martinez told evertontv.
"It was important to get another goalkeeper with the quality of Joel on a permanent basis. He has experience in the Premier League and was part of the squad that won the European Under-21 Championships with Spain.
"He is someone who is going to give us great protection and fight with Tim Howard for the goalkeeping position and push him all the way.
"He comes from a rich vein of successful goalkeepers. David De Gea, as you well know, came to the Premier League - he and Joel grew up together. Atletico Madrid's school have developed some outstanding youngsters and I was delighted to see him performing in the manner that he did (last season).
"To bring him across at his young age and see how well he adapted to the British game, he ended up being Wigan's number one goalkeeper and winning a major trophy. That's the aspect that impressed me more than anything."