Parker has started five of the last seven matches since impressing as a second half substitute against Denmark in Copenhagen in February.
When asked if he thought he was guaranteed to go to Poland and the Ukraine next summer, Parker said: "Not really. Football has chucked up a lot of surprises in my career.
"There is no complacency. I realise how tough it is going to be. I need to retain my form.
"But I've come a long way in six months. Six months ago I wasn't even really involved."