With all their expertise available and a previous record of fast change, I doubt it would take long for them to catch up if they have lost some ground in terms of aerodynamics. The suggestion Sunday was around a significantly improved Honda engine. I would have thought it might take longer to match, but would like anyone well versed in these things to explain what might be concerning Mercedes most.
It's a combination of factors. The aero reductions that affect the rear of the cars in the new regulations appear to be hampering cars that run a low rake (Merc/Aston) more than those that run a high rake (Red Bull, Alphatauri). Then you can factor in the new Honda engine which seems more powerful (a bonus in itself) , but more importantly is more compact meaning Red Bull can go further with aero at the back of the car than Merc with no physical change to the engine size. Add in Max Verstappen (who genuinely seems fast than any of his teammates by a large margin) and you have a faster Red Bull.
Luckily Mercedes have Lewis Hamilton, but they are worried that their car's high rake approach may not allow them to make up the lost ground to Red Bull especially as the cars as we know them are only here for one more season and next year the regulation changes are massive so at some point they have to start focusing on 2022 although that is the same for all teams. I think we will see some ebb and flow between Merc and Red Bull depending on the circuit types, but if Lewis is to win number 8 I think he is going to have to work harder than ever.... no bad thing for us, the spectators.