The Moroccan's moment of magic teed up the Blues' 2-0 win over Spurs on Sunday, their third victory against their London rivals in this month.
Thiago Silva's header cemented the west London club's first league win in five outings since their success at Aston Villa on Boxing Day.
Ziyech, though, was the star of the show on Sunday evening as he ran Spurs ragged on the right flank of a 4-3-3 formation.
Speaking on Sky Sports, Keane admits the 28-year-old has failed to become a starter as he has never produced a string of impressive displays.
"I think the reason he doesn't play regularly is because he's inconsistent," Keane told Sky Sports.
"He set a good standard today and scored during the week, but he needs to be in line with it.
"To play in one of the big clubs, you have to play weekly and weekly, as talented as he is, but he doesn't play regularly."
Meanwhile, Tuchel has told Chelsea to kick on with a good run of form after the Blues put their bad run of form to bed with a fine victory in the derby.
"The attitude, mentality and spirit of the team was very good on Saturday in training, you felt that they wanted to prove a point," said Tuchel.
"I continued to have a good feeling before the match. I saw them and how they were together and prepared the match and I am happy they could prove it.
"Of course Hakim's goal gave us a massive lift, but I am even more impressed because we didn't score that goal after five, six or seven minutes or the early stages of the match, but went from there with pure, positive emotions.
"We worked hard for it and when it came in the second half we kept on going and kept on pushing and kept on believing, so that was very good because we came from a lot of games and some results we didn't like.
"And it was a positive match and a good performance and a clean sheet. It was very good.
"I think the fact it was a London derby helped us and now we have some days off and the players knew it also helped us because it gave us a feeling like a cup game.
"Now we go into a break and it was important to go into this break with a victory. There's no doubt about it because if you don't have the results and the performances you want, there's no substitution for a win and that feeling.
"You cannot produce it artificially so we needed this but we needed also to do things better and keep on believing. The guys are very tired now because it was a long run but it was important for us."