Yeah, they made his base HP regen comparable to Rivens, his is literally only** 0.07** higher than Rivens 0.07. And unlike Riven, he doesn't have earlygame cheese, nor is his sustain nearly as easy to buy into (Riven can build rejuv beads at the start of the game which go towards her tiamat, and vamp scepter basically solves most of her sustain issues, whereas mord has to build a hextech to gain spellvamp, which is much more expensive than the cost of vamp scepter + 2 rejuv beads, and the sustain that hex provides is definitely not as strong as the sustain that a vamp scepter with 2 rejuv beads will provide). So basically, they made his earlygame even WORSE than before.
Before, it would take him 14.9 seconds to regen back to full health from using his Q once (barring masteries and runes, and also assuming to be at level 1), now it will take him 29.6 seconds. That is almost 15 seconds longer than it was before.
Here is the math:
NOTE All calculations are assuming mordekaiser is level 1, and is not using any form of health regen masteries or runes.
OLD:
8.37 / 5 = 1.675 health regen per second
25 / 1.675 = ~14.9 seconds to reach full health
NEW:
3.37 / 5 = 0.674 health regen per second
20 / 0.674 = ~29.8 seconds to reach full health
These costs are using the new values for his Q and W, not for his E which is generally his most used skill during the laning phase, which had its cost remain the same, which makes the base health regen nerf even WORSE. Then you have to consider the fact that not only does mords own spells hurt him, the enemy is bound to be hurting him as well, which he will have an even harder time recovering from.
Mordekaiser was nerfed heavily by these changes, and will require some pretty big buffs to have a hope at being competitive.