His new manager at Pittodrie, Jim Goodwin, wants more competition in the goalkeeping department than last season, with the 2021/22 No 2 Gary Woods having left the club last week.
Lewis is the current club captain, but after the arrival of Roos, the Englishman is not guaranteed to be a regular starter in the upcoming campaign.
Speaking from Aberdeen’s training camp in Spain, Roos said: “You come to Aberdeen, it’s a big club, and big clubs have good players and they have to challenge for places.
“I’m challenging, Joe is challenging, everybody on this pitch today is challenging for a spot.
“I want it to bring the best out of me.
“You spend a lot of time together. Keepers quite simply have to make the choice: ‘are we going to make it awkward all year round or are we going to work together and accept decisions about our position aren’t up to us?’
“It’s very simple. You’ve got a gaffer, you’ve got a goalkeeping coach. I would say most of the time, the gaffer decides who plays.
“That’s not up to me, it’s not up to Joe. It’s as simple as that.
“For me, it’s a case of working together and performing to the best of my ability every day and hopefully get picked.”
Roos, 30, made 18 appearances for Derby last season as they were relegated from the Championship.