It's actually bad news for us.They will be very well rested vs us while we play Atletico. Shame shame United!
Does not matter much in the wider picture. There are 4+ days between that Atlético game and our first leg. By the time that we will play Atlético at the Camp Nou, the league might be practically over with us leading with potentially over 10+ points.
Teams like Bayern, PSG and Juve have almost often had their own domestic leagues wrapped up in the CL knockout stages and thus had amble of opportunity to rotate but it almost never helped them. Often, in fact, it hurt them as it is difficult to switch on after not playing a competitive game for a while. See our Roma game last season as well when we only had the CL to focus on as the league was wrapped up as well as us booking a place at the CdR final.
Having 8 days of rest is not necessarily a good thing. I prefer us to be playing regularly up to big/important ties. Provided that Valverde will rotate which he will have no other option than doing after the NT break as we will be playing 3 league games prior to the first leg against Man Utd. The same Man Utd will play twice in the span of 4 days (30 March and 2 April). So more or less the same with the exception of the Atlético game where we might rotate heavily or at least significantly depending on our lead. If not it is 4+ days of rest. Don't see it as much of a problem IMO.