First of all; Aguirre isn't really a tactical genious and I think most Atléti-fans knows this by now. The guy rarely does anything that changes a game and he probably won't be doing tactical wonders with the current squad either.
Atlético are playing poor football at the moment, mostly because there isn't any communication between certain parts of the team. Let's start with the defenders; They're not good enough to take on pacey players and need support from the defensive midfielders, Paulo Assunção and Maniche in this case. The first one always helps out and plays as a third CB at times while Maniche rarely goes down to help out the defenders, he just stands there and there's a big difference between this version of the player and the one that played in the beginning of the season.
The back four isn't communicating good enough, I can understand if certain players, such as Heitinga is having a hard time to understand his teammates, but it doesn't explain his individual misstakes at times, the guy has conceded three penalties so far. Our defenders are very nonchalant, they're not enough resolute, they feel that it's not acceptable to kick away the ball in certain situations. They're not composed either and the confidence is poor at the moment. Aguirres task is to help them to regain it.
This team is playing with more or less four strikers, they're rarely doing some work defensively as they're supposed to be attacking. So, Atlético has basically six defenders and four attackers. It's important to re-organize against Barcelona, the team may not be able to knock them out, but give them a chance to show that they're capable of playing a lot better than they've done since the start of this year. Bring back the confidence Atlético had before playing Valencia, the players haven't lost all their talent all of sudden; This is a very good team offensively and they can definitely challenge for a Champions League-spot.
Let's be honest with ourselves though; There aren't any better replacements at the moment. I hope Atlético holds on to Aguirre until the end of this season and talks through things then.
I've always found Maxi to be heavily overrated. I don't see what's so special. Not very fast, not a great dribbler, not a great passer, not a big time scorer... I don't get it.
He's not suited to play as a winger, he doesn't have the attributes and I personally think he'd do a lot better as an attacking midfielder in terms of the way he plays. He's an important player for Atlético though, the guy always gives his all even though he isn't playing that well.