Past two games he's made some poor decisions. Most notably that of playing Puyol at FB and picking Masch over Bartra. Did that affect the result? Yes. Did that affect every possible negative observation we've seen? No. We could've kept from conceding the stupid goals that we did, but honestly both Ajax and Athletic played monumentally great games and it would have been difficult no matter what.
What Tata needs to learn is to primarily break his decision to "respect CB pairings" because Masch does not deserve to play over Bartra. His substitutions could also be much more decisive and come earlier. Other than that, not much he can do in an injury plagued period like this one. Cesc as false 9 isn't good but it's less of a loss than Neymar or Alexis there. Both of our starting fullbacks are injured, not to mention our goalkeeper and Messi himself. All of that, coupled with Neymar's current slump and Xavi's progressing age makes it pretty difficult to compare to the "glory Pep years" that everyone is using as a standard. It's a bad spell, it happens, but there's too many things happening that are just down to bad timing or bad luck for it to be a problem solely caused by Tata (he hasn't done well recently though).
It's quite obvious that signings need to be made in January and further in the summer. For January I'd say just the CB is needed and MAYBE a striker depending on what we see on Messi's return. In the summer, it's got to be ter Stegen and a CM. Xavi is out of it now, and our midfield lacks engine. I fear that the club will be too conservative with transfers in January and refrain from doing anything, but that's unfortunately the probable outcome.