Foden had asked not to be included in Tuchel's squad named yesterday, but the England coach says his form this season was also a concern.
Tuchel told talkSPORT: "For Phil it is a little bit different because Phil had a difficult time since last camp.
"I loved Phil as a person in camp, he was fantastic. I love his talent and the ability that he showed in training with us. He was simply outstanding, there is no doubt about it.
"He struggled even before last camp to show it in the club. He struggled within camp to feel the freedom and the excitement that is part of his game."
Omission a mixture of things
Tuchel also explained: "Things were not so easy for him. They became a bit more complicated after the Manchester derby and on a personal level what happened in the Manchester derby with the fans and then also on a physical level that he is still suffering from ankle problems, cannot get rid of that.
"So putting it all in the mix we thought it better, it is the moment now for Phil to take a breath, to not be with us, which was also a very hard call because I admire him a lot and I like him a lot.
"But we thought it is better he takes care about his ankle, gets fully pain free and has no reason for holding back and be fit for the Club World Cup. So it was a mix for Phil between injury and performance wise."