I don't think 3rd and an FA Cup would be underperforming with the players we have but it is underperforming with the money spent. I don't think much of the latter can be blamed on the manager as its been a problem for a few years.
Apart from Stones,non of our defenders would get into the side of another top 6 team. The full backs would struggle to start for teams playing mid and lower mid table.
Players like Kompany, Zabba, Yaya, Clichy, Kolarov aren't even close to the level when we won the league and the new players we bought in mostly aren't close to being as good as that older guard was.
We seem sorted in the forward areas now but keeper, defense and defencive mid is not even close to being title winning level.
I've seen enough this season to know that when that back line is sorted we'll be the top dog again.
I'd also point out as well that major game changing decisions went against us in home games involving the top 4 and we would surely have won them all of they were correctly called. These decisions stopped players from scoring goals and being at least clean through. Chelsea also had the huge benefit of not being in Europe as well.