The Tangerines head north knowing a win would take them to within just a point of Terry Butcher's Caley Thistle 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 the former Partick boss 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.
"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," McNamara added. 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.