Robertson said several players, including All Blacks midfielder Jack Goodhue (knee injury), didn't travel to Queenstown.
"We've got a little hub back in Christchurch; they're training away. We've got a coach and a [strength and conditioning coach] and a physio. They've got some rugby components they can get in," Robertson said.
"The big challenge for us is giving them some rugby, so they can get ready before we come back into round four. So we're managing that from afar."
Players joining the Queenstown bubble will have to self-isolate for five days in a hotel room and return three negative Covid-19 tests.
"For someone to do that, it's a big call, hence we brought four guys on top of our 35 to give us depth and cover our All Blacks, and a couple of injuries," Robertson said.
All Blacks George Bridge, Braydon Ennor, Scott Barrett and Joe Moody have been named to play in the Crusaders' final pre-season clash on Saturday.
The Crusaders' six other healthy All Blacks will be available for their season opener against the Hurricanes in Dunedin.
"They have been training hard. They've got to get their own contact ... but when you've got a Sam Whitelock, who is a wily old, crafty player, he will get himself right," Robertson said.