In the match against Udinese, Inter Milan still haven't found their original form completely, when against Udinese they managed to concede 2 goals even though in the end Inter won by scoring 3 goals, but it seems that Inter's defense line this season is not good, they have conceded 7 goals during Serie A this season, in my opinion if they are not so solid the next day for an important match for their points in the Champions I think they will be embarrassed by the Serbian representative club in the Champions.
In my opinion with their impressive last season, Inter are getting more solid and can be impressive again this season to compete for the Champions but it seems they are a little weak now and I think they need time to recover and I am pretty sure Inzaghi will do it for Inter.
Yes, defense has definitely been a problem area for Inter Milan this season. They have conceded more goals than they would have liked, which shows the instability in their backline. This raises concerns especially since they are about to face critical matches in the Champions League.
Ideally, a team that wants to compete for all major trophies needs to be solid in all areas - offense, midfield, and defense. But as you suggested, it takes time to gel as a unit, especially when there have been some significant personnel changes.
Let's hope Inzaghi can indeed find the right balance soon because the potential of this Inter team is very promising, as was evident in the last season's performance. They have the talent and they have shown before that they can play effectively together, they just need to find their form again.
Whether they can do it in time for the Champions League matches is something that remains to be seen. But the hope and expectation certainly is that they will.
I am guessing aging has something to do with it, and also they are not hungry anymore, that plays a big role. They already won what they want to win and they even went to UCL finals which is hard to do and they may have lost there but they know they will probably never get back there, so Serie A is the only realistic thing and they already won that, so they are not feeling hungry anymore at all. They are like Rocky against Clubber Lang on the first fight.
When you are that fulfilled, you do not play with 100% and you end up being so much worse, and this season they look like that. They have the skill and the talent to beat teams of course, like they won against Udinese which shocked nobody, but 3-2 was a risky score, could have been a tie very quickly, they got lucky that they ended up in a win.
Teams who win a lot, do not continue to win a lot normally, we see PSG and Bayern and think that's it, but there are many cases all around the world where a team does very well for a few seasons, sometimes just 2, sometimes as high as 5, but then they slowly start to be terrible, not right away in a single season, but they start to be terrible eventually. A great example at the moment is Lyon, this is the team that won so many in a row, and now they just suck.