Under fire Sydney Roosters' coach Trent Robinson has explained the decision to bench Brandon Smith for the club's game against the Penrith Panthers on Friday night.

The trip to the foot of the mountains ultimately ended in a disastrous 48 points to 4 loss for Robinson's outfit, with questions arising over everything from selections to the Roosters' effort and execution at both ends of the park.

The big surprise pre-game came an hour before kick-off when Robinson elected to promote Jake Turpin to the starting team, while Smith found himself back on the bench for the contest.

Smith has struggled to adapt to full time dummy half duties at his new club after moving during the off-season from the Melbourne Storm, and Robinson explained saving interchanges, as well Smith's energy in the second half, were behind the surprise switch.

"I have been having to make multiple changes around that position there," Robinson said.

"I wanted him [Brandon Smith] to come on and create that momentum.

"Turps [Jake Turpin] has been playing really well, so I wanted him to set up the game, and then to have Smith come on and be fresh coming out of halftime and add some speed as well."

Turpin, who himself made a switch from the Brisbane Broncos where he was unwanted during the off-season, has made five appearances prior to Friday night's game for the tri-colours since the move south, with only two of those previous appearances amassing to more than 50 minutes.

Despite that, he has tackled at almost 99 per cent and provided strong service, with Friday's game seeing him play 27 minutes with 26 tackles (0 missed) and an offload amongst his stats line.

Smith played 55 minutes in a single stint, but struggled to influence the game and also came up with four missed tackles in a dismal defensive effort from the Roosters across the course of the 80 minutes.