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The Curiosity vehicle is aiming for a deep depression known as Gale Crater.
The US space agency will be tracking the descent with satellites, but its prime orbiter for the task may not now be in the correct place in the sky.
Engineers have been tackling a fault on the Odyssey satellite and it has yet to take up the best observational orbit.
Unless that is achieved in the next three weeks, Nasa will lose signal to the rover just as it is about to touch down.
This will not affect the outcome of the landing because Curiosity's descent manoeuvres are all performed autonomously, but it will give rise to some high anxiety as everyone awaits confirmation that the $2.5bn mission is safely on the surface.
"To be clear it won't have any impact on landing; it's all a communications issue," said Doug McCuistion, the director of Nasa's Mars exploration programme.
The 900kg robot's entry, descent and landing (EDL) will be the most dangerous aspect of the entire mission.
The rover, in its protective capsule, will hit the top of the Martian atmosphere at 20,000km/h (13,000mph) and attempt to slow to just one metre per second to execute a soft touch-down.
This rapid deceleration must be achieved in about seven minutes or Curiosity will smash into the ground.
Engineers have built a complex EDL system that includes a supersonic parachute and a rocket-powered crane. Everything must work on cue and in sequence.
It was expected that the Odyssey orbiter would track the whole descent, relaying UHF signals from the rover right up to the landing and for a few minutes beyond.
But the spacecraft recently experienced a reaction wheel failure.
This device is used to keep the satellite properly orientated, and because engineers have been investigating the issue they have not as yet moved Odyssey into the correct orbit to see the full landing sequence - and they may not do so.
This would leave Nasa blind for the final, nail-biting two minutes of the landing operation.
The agency will have two other orbiters watching the decent - the Mars Reconnaissance Orbiter, and the Europeans' Mars Express spacecraft - but these only have a "store and forward" capability, which means they will not be able to confirm a landing has taken place until their data has been passed to Earth. That will be some hours after the event.
Antennas on Earth will be no use either in the last moments of landing. Gale is one of the deepest holes on Mars and the steep crater walls will block all radio transmission from the rover.
As things stand, and unless Odyssey is moved, Nasa will have to wait until the satellite in its present orbit passes the landing site. This could be five to 10 minutes after the planned touch down (22:31 PDT 5 August; 01:31 EDT, 05:31 GMT, 06:31 BST 6 August).
Curiosity - also known as the Mars Science laboratory (MSL) - is the most sophisticated space vehicle ever built to touch the surface of another world.
Assuming the robot lands safely, it will spend 98 (Earth) weeks scouring Martian soils and rocks for any signs that current or past environments on the planet could have supported microbial life.
Gale Crater was chosen as the landing site because satellite indicates there are sediments in the depression that were laid down in the presence of abundant water.
MSL-Curiosity is equipped with 10 advanced instruments. It also has a plutonium battery and so should have ample power to keep rolling for more than a decade.
It is likely the mechanisms on the rover will wear out long before its energy supply.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter
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