Depends on situation - the NLAW has advantages (when used properly) in a lot of the fighting Ukraine has been doing - close to medium range, high mobility, medium contact engagements and pop up and run. While on the counterattack or for longer range non-direct-contact engagements they aren't a good substitute for the Javelin though.
No not just not a good substitute, not a substitute at all in that case as they don't have the same range, yes there are situations where the NLAW has some advantage, especially at very close range in an urban environment but overall the Javelin is a way more advanced anti-tank weapon.
In the British army for example you'll have, within an infantry battalion, a support weapons /maneuver support company containing recce, mortar, machine gun, assault pioneer and anti tank platoons.
The anti-tank platoon will have Javelins, they specialise in... well destroying tanks and armoured vehicles. Back in the day they'd have had MILAN, they're fun to watch go down the range at Otterburn, each missile is about the price of a Golf GTI.
Regular rifle platoons on the other hand will have NLAW when appropriate, back in the day it was LAW 80, unlike the anti-tank platoon guys it wasn't so common for people to get to fire these (though in the case of the LAW 80 there was an inbuilt 9mm spotting rifle that could be fired during live-fire demos - ironically I was involved in doing one of these in the 00s for some Ukrainian SF types!).