This will for sure mess up the constellation, which is designed to minimize the times where some places on Earth do not have 4 satellites above the horizon, and also the places where this is going to happen (i.e., coverage gaps over the far South Pacific are likely to be more acceptable than over Northern Europe) . Since these satellites are too low, they will have shorter periods and will thus not be commensurable with the existing constellation, and will drift in and out of place.
You can be sure ESA engineers are busily looking at orbits this weekend, to see what can be salvaged from this debacle. Now, they may be really lucky, and have gotten an orbit where these two satellites can be used to fill a hole in the current constellation. I would bet in that case that both satellites would serve to fill the spots normally filled by one satellite; so at best only one, but if (as is more likely) they are unlucky, two satellites will have to be launched to fill the gaps.
In other words, while these satellites are not a loss, and will be used, new launches are likely to be necessary to make the constellation whole, which will cost as much as if they were lost.