I don't. First of all, they're not going to shoot one missile at an incoming ATACAMS targeting anything of significant value - they would (if in a position to do so) shoot at least two for insurance purposes. I think the 2+ missiles would cost Russia, as a percentage of it's GDP, staggeringly more than an ATACAMS would cost NATO. I'm not sure I buy the idea of Pantsir as an ATBM system - I suspect the Russians would use more capable and long range missile systems vs it, given the chance.
Sooooo... Your bet? Sit and wait for impact?
No. Effective deterrence is a matter of convincing a potential attacker that their offensive action may well not achieve its aims and/or that the cost of achieving those aims will outweigh the advantage gained by achieving them. Clearly conventionally tipped TBMs (which Russia has also invested in) are not infallible, nor are they, in themselves an effective deterrent. However, the fact that you have those things in your toolkit forces the opposition to take them into account. I have great respect for Russian military capabilities - not just in their scale, but in the intelligence and imagination put into them, leading to them differing in many ways from those of the west. I believe, for example, that Russia could easily take out all the military airfields within at least 1000km of its Western Borders with survivable standoff weapons. For that reason, and Russia having best-in-world GBAD/IADS, I think Romanian investment in high performance aircraft, beyond a small number required for airspace policing, is a massive waste of money. Those 54 HIMARS MRLS on the other hand could easily disappear in the complex terrain and built environment of Romania or other E. European countries and pose a standing threat. Arguably their GMLRS capability is more important (definitely more survivable and offering greater target defence saturation capability) than ATACMS, but both can easily be replenished by Uncle Sam and GMRLS by other NATO allies. This is a capability that Russia cannot easily destroy whilst in the field. SAMs may neutralise them to some extent, but you have to realise that there will be a whole lot more going on than ATACMS vs medium/long range SAM shootouts. Will Russia want to keep SAM and EW radar radars transmitting on the offchance of an ATACMS attack? Will those SAM systems themselves be under attack? Will there be decoys, jamming, stay behind teams etc.? Will they be logistically resupplied? Will their command and control network be compromised or taken down? The chief problem for me with systems like ATACMS would be long range target acquisition - obviously less of a problem with GMLRs (and its Russian counterparts). Given the expense lack of survivability of other methods (the Russians could even blind recon satellites with ground-based lasers), I think it would probably come down to stay behinds and agents with satphones and GPS/GLONASS.