Aston Villa: Martin O'Neill demands focus after bust-up with Nigel Reo-Coker
"Some have gone and he has had to make changes but if their effort and determination last Saturday is any guide it will genuinely be tough for us.
"As I've often said in the past and will continue to say we wouldn't be good enough to treat anyone lightly.
"But we are going into the game with a lot of confidence."
Villa have made their best start to a Premier League season since 1998 and O'Neill added: "We have won three consecutive Premier League games and it is great.
"Portsmouth, however, will be dangerous as they showed last week from set-pieces and counter-attacks.
"I just think that we have got to be really focused on the game and I think the crowd can be encouraged by our recent performances."
O'Neill could recall Fabian Delph to replace Reo-Coker in the heart of the Villa midfield and he has also has not ruled out a return for striker John Carew to join Gabriel Agbonlahor in attack.
Carew, a firm favourite with the Villa fans, created the winning goal for Agbonlahor at Birmingham last weekend after going on as a substitute.
O'Neill added: "We will make assessments accordingly. I will give it some consideration and have a look at it.
"I made some changes against Birmingham but it was imperative that I got a couple of defenders into the side as it gives us a bit more stability considering that Curtis Davies will be out from a lengthy time after his shoulder operation.
"We are reasonably flexible within the system we play.John Carew came on and made the winning goal at Birmingham which was delightful.
"At least I now have some options and I have players chomping at the bit which is encouraging from my view point."
New defenders Richard Dunne, James Collins and Stephen Warnock, who made their debuts at Birmingham, will all make their home bows as Villa players.
