United's former first-choice goalkeeper Andre Onana, who endured a turbulent spell at Old Trafford, completed a loan move to Turkish side Trabzonspor on Thursday.
The deal followed United's deadline-day signing of Lammens, 23, from Royal Antwerp for 18.2 million pounds.
Turkey international Bayindir, 27, has started all three of United's Premier League games this season, while the 29-year-old Onana only played once, in United's shock second-round League Cup loss to fourth-tier Grimsby Town in a penalty shootout after the sides drew 2-2. Onana was at fault for Grimsby's two goals.
"It is a different league, a different country, different training, different ball. So we will try to maintain that and they will fight for the position but for me in this game is clear, Altay will start," the Portuguese coach told reporters.
During United's League opener against visitors Arsenal, Bayindir misjudged Declan Rice's 13th-minute corner, palming it onto the line for Riccardo Calafiori to head in.
"I know that we are in a moment where the goalkeeper needs to be really strong and have a lot of experience. We are also in the moment where we have to look at the present but also with a focus on the future," Amorim said.
"Lammens is a guy with a great potential that is one more option (for us) to play, and he will be ready."
Amorim also explained why United chose Lammens over Aston Villa's Emi Martinez. "We have an option that can give us different things in the moment but also has a great potential to be our goalkeeper for a lot of years so that was our choice in the end," he said.
Amorim confirmed that new signing Matheus Cunha, who has played all four games in all competitions for United, will miss the derby through injury, as will Mason Mount and Diogo Dalot.
"I don't want to say to how long they will be out for. If you talk to Cunha he will say he is able to play this game. So I don't know. We will manage day-by-day. They are pushing really hard and that is a good thing."