Viduka only had a two-week break in the summer because of his international commitments with Australia and Allardyce feels those games, have drained the player badly.
Toon Striker 'Pays The Price'!
Allardyce: "Mark is struggling and he won't be involved against Portsmouth, which is obviously frustrating for him and us.
"I think he's paying the price for a long, hard season with Middlesbrough and then going off to play for Australia in the Far East over the summer.
"I think he's paying the price for a long, hard season with Middlesbrough and then going off to play for Australia in the Far East over the summer.
"That appears to have taken a lot out of him and that is why he has struggled to stay fully fit. It was a demanding schedule for him in the summer in
difficult conditions and, unfortunately, we have paid the price for that.
"The Sunderland game would be a good one for him to return for, particularly if he scores. I don't see any reason, at this stage, to think that he won't be available for that because he's pretty close now.
"We've maybe brought him back a little quickly in the past and, by giving him another week's training before picking him, we hope that will mean he's in better shape in the long term."