"Well that was an absolutely okay performance," Tuchel told reporters at Stamford Bridge after his side lost 1-0 to Arsenal.
"We saw they defended so deep and we wanted to have Mason who has a better shot from 18 metres to be more dangerous from distance, have an offensive player like Mason there.
"It wasn't that Billy was bad, we just wanted an offensive change for tactical power. For Billy's situation, we will talk after the season but right now he is here and doing his job. It is clear that it is very hard for him.
"Today, we lacked N'Golo and Mateo Kovacic in midfield. It is unfair to put it all on his shoulders and he had a fantastic game against Manchester City. It was an okay match today. He was by far not the problem."