Redknapp was admitted to hospital on Tuesday night to have tests after he started feeling breathless while running.
The Spurs boss had two stents inserted the following day to help unblock two coronary arteries and he was ruled out of the club's Europa League trip to Rubin Kazan.
Redknapp, 64, will now rest for the next two weeks and assistant manager Kevin Bond will continue to manage the team in his absence.
"We have to win because he is not well at the moment," Gallas said. "The manager is a good guy so I think we have to do a good job on Sunday for him. We have to do it for the squad and the fans too."
Last season, Tottenham fell six points short of qualifying for the Champions League following a mid-season stutter, but Gallas, who was a regular in defence last term, thinks the squad has a better chance of finishing in the top four this year.
"The squad is stronger this year," he said. "We believe we can finish in the top four and maybe win something.
"You never know in football. Everyone is talking about (league leaders) Manchester City. They are doing well, but I think we have to believe in ourselves and try to win every game and at the end of the season we will see."