Sanchez said: "Kasey kept a clean sheet, which was only the second one we've managed this season.
"I made a decision against Chelsea to go with Kasey, and he didn't let in any goals, which is all you can ask a goalkeeper to do.
"Antti Niemi is being plagued by the wrist that he injured in pre-season, and it still isn't 100 percent right, but the good thing about having three goalkeepers (Keller, Niemi and Tony Warner) is that it gives us good competition.
"Some players might not like that, but the facts of life are that you need competition if you are a Premier League club.
"Antti was fantastic for us last year, and I have said before that we would have been relegated without the saves he made late in the season.
"But that was last season. With one thing and another, he wasn't fit for pre-season, and then he was injured in the warm-up at Arsenal (on August 12).
"Unfortunately, you can't wait for ever for players to be right. You have to look at alternatives, and if you play well, you deserve a chance.
"Kasey has the shirt, Kasey played on Saturday, and he kept a clean sheet, so it is more than likely that he will play against Portsmouth.
"But that is what football is about. If you play well, you usually stay in the team."