At the macro level the key performance requirements have been met:

But looking purely at match results, the team is not improving. Pts% is calculated assigning 3 points for a win and 1 for a draw to assess every result. It is far more powerful than Win% which completely disregards draws.

My data collection started in the 14/15 season so taking the last 5 years, the 16/17 season is starting to look like an exception sticking up defiantly like a middle finger!
The Invincible domestic record may never be broken and was spectacular. But either side of that Pts% is a consistent 70%.
Based purely on match results, and non-believers say this is the only statistic that counts, Celtic are not improving and results-wise are no better than under Deila.
The trophy haul in the Rodger’s era is, however, exemplary. Seven domestic trophies in a row – 100% record. Deila managed 2 SPFL titles and 1 League Cup. It could be that Rodger’s is better at getting the team organised for the big matches and is certainly a Cup master.
So, we shouldn’t be concerned by current result trends providing Celtic continue to deliver when it counts.
How Deila must rue Steven McLean though.