The Tangerines are relying on rivals Dundee to do them a favour against Kilmarnock if they are to secure a top-six slot going into their final pre-split fixture with the Dons.
McNamara said: "It's one of those ones where results elsewhere could dictate things, but we can't afford to think like that.
"We only ever focus on our own thing and that won't change with this game. We know things could still change and we have a chance of getting into the top-six.
"That's all that matters as far as we're concerned. We will never give up. As long as there's a possibility we will keep going and that's how it has to be."
McNamara is hoping for the United fans to turn out in numbers and play their part in the contest.
He added: "We want to give the fans a good end to the season.
"We need their backing which the players appreciate. They can certainly make a difference and haven't been able to celebrate enough home wins this season.
"We want that to change in the home games between now and the end of the season, and there's no better time than against Aberdeen this weekend.
"These are always tough games. Aberdeen are well-drilled under Craig Brown and we know it will be a very hard 90 minutes."
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