The transfer fee reflects the players' value in the transfer market. A core aspect of a player's value are his footballing abilities. A player worth a fee of 10 million obviously doesn't have the abilities of a player worth 89 million. Hence you can expect a player worth 89 million to at least have the impact of an average player. Nobody is asking Pogba to perform on his highest level, but given his abilities, one can expect him to perform on the level of an average player, which he hasn't done. He had a good start against Southampton and has been on a decline ever since.
Your analogy is poor because the value of a player that is reflected in his transfer fee doesn't include super healing powers.
N'Golo Kante only joined Chelsea onthe 29th of July for his first training session, which is just a little over a week before Pogba joined our training, yet Kante has been very good for Chelsea according to Chelsea fans.
I can understand a poor start in the first couple of games because you have to get accustomed to the manager's tactics and return to proper footballing shape, but it's been several games since and for him, the trend is going down rather than up.
Martial's first game came on September the 12th because he only joined us at the beginning of September. He may have had more footballing training beforehand, but he was hardly familiar with our tactics.
Ultimately, all of this is not to say that he wasn't worth his money or that he'll be a flop. I firmly believe he'll be a success here. But there's no reason to excuse his performances so far and claim his transfer fee is irrelevant.