"He didn't like what F1 had become and was quite vocal about it - didn't like the engines, the noise, the way the car felt," Horner said.
"He was pretty upset about that at the beginning but once he realised that was it - it wasn't going to change - he started to get his head around it. He worked very hard, as he always has.
"But
he got very frustrated the car wasn't doing what he wanted it to do and of course to compound that his team-mate is winning a couple of races and performing at the level he was."
"The rule changes were significant and the two guys who seem to have been affected most are Seb and Kimi and I think
taking away the rear downforce has actually been a big factor for Sebastian," Horner said.
"
How he generated his lap time was very much using the rear of the car on entry into the slower corners, which was much diminished this year, and that together with the brake-by-wire system had taken away some of the feeling of the braking that he is so dependent on.
"He is such a 'feel' driver that some of those feelings had been muted and it dumbed down some of his performance."
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