The Tangerines head north knowing a win would take them to within just a point of Terry Butcher's Caley Thistle in their quest to climb the table as they look to finish the season with a flurry.
McNamara watched his side concede two goals in last Sunday's 2-2 SPL draw with Hibernian at Tannadice, a game in which United required a late penalty from Johnny Russell to earn a share of the spoils.
And McNamara does not want to see a repeat of any slack defending when they head to the Highlands for their important SPL clash.
McNamara said: "Hopefully we have learned our lesson from that point of view. It's something we have to work on and have got to do better. Inverness have been playing well this season and thoroughly deserve to be where they are in the league, which is second top.
"They score a lot of goals so obviously we want to keep things tight at the back and not help them in any way. We were annoyed at the goals which we lost against Hibs but that's in the past and we have to try and improve on that."
McNamara takes his side to the Highlands to face Euro-chasing Inverness and knows they will have to be firing on all cylinders if they are to return to Tayside with all three points. He said: "We know it will be nothing other than a difficult game.
"It will be a test for the lads and we have to start the game better than we did against Hibs when we were slow to get going. Having watched Inverness, it's clear they have had good legs and can get about the park so they are dangerous opponents.
"But we have to focus on our own strengths and hopefully take the game to them. The league is so tight that a few wins can make a big difference which is what we're aiming for."
United have signed Swedish defender Marcus Tornstrand but the 23-year-old free agent will not feature at Inverness as he works to build up his match fitness. Gavin Gunning is sidelined following a knee operation but McNamara has no fresh injury worries.