it doesn't remove the risk, it raises the reward.
how is he comparable to an assassin? assassins have much less trouble getting past tanks than garen, and can kill a squishy twice as fast.
this is hardly an offensive change. let's say it's 5 seconds of health regen uninterrupted, as per his passive. it's only 2% of his health before level 11; so even if he gets to say 3000 health before then, it's only 60 health regen'd,. it barely affects his early game but lets his im temper off his immense falloff later on with the chance to make a good pick and refill his health bar. then we can let him have a stronger heal for killing the villain, to give him chance at even bigger payoff since the villain is usually the carry or a mobile assassin.
the way I see it, this isn't that terrible an idea; it gives him in-fight sustain like every juggernaut has, but is conditional on being able to kill someone with his ult and is not % missing heal. It's also a HoT, meaning if he uses it while low, he's still vulnerable to getting bursted down before he heals back up