"We needed an in-form David Havili at the back, and Seta can cover centre and George Bridge can cover fullback, and Manasa has been incredible off the bench for us.
"So you look at Dagg - where's he going to fit in? Obviously, with his knee and injuries, they haven't been timely and he hasn't got his rhythm, so we've gone for our in-form players."
Robertson says Dagg will get another chance before the Super Rugby regular season ends, and he'll need to take it to force his way into the table-topping side.
"He was pretty gutted, but when you've got in-form players, you know, George Bridge gets the start on performance - this is one of the players of the year, one of the top try scorers in the competition, he's been absolutely phenomenal, especially in these derby games.
"So when you have a conversation and you explain to Daggy all these things, he can obviously say 'Look, I need to perform next time I get an opportunity,' and that's what he needs to do."
Dagg is set to play for University on Saturday in club rugby, and Robertson says it will be good for him to get his rhythm back.
"He hasn't actually played many games this year at all, he still needs game time, so club rugby's the place for him at the moment."
Dagg isn't the only big name to miss out on selection in the Crusaders 23, with Tim Perry, Luke Romano and Pete Samu also sitting out the clash.
