Hamilton has won six of his record-equalling seven world championships with Mercedes, although he has not yet committed to the Silver Arrows for the coming campaign.
A resolution is expected soon, but links to rivals Ferrari have never been far away.
And team principal Mattia Binotto is confident they have made the right move.
He told Sky Sports: "I don't think there will be regret because, at the end, when we took some decisions, we took them thinking they were the right decisions and today we have a fantastic driver like Charles, where we invested a lot as Ferrari.
"I think he's got a lot of talent. If he's got the right car, I'm pretty sure he can challenge Lewis Hamilton.
"We've made our choices and I think, together with Carlos, we are very strong. I don't think there will be any regret."
Although Leclerc - fourth two years ago with two wins to his name - would appear the obvious Ferrari contender, Binotto is not naming a "number one".
Sainz finished seven points better off than his new team-mate in 2020 while still with McLaren.
"There is not a number one and number two," Binotto said. "They will have equal opportunities - certainly at the start of the season. I'm pretty happy then that they are free to fight.
"I think what we have to do as usual is to optimise the team points, and when you optimise the team points, normally you are optimising the drivers' points as well. Team points is the number one.
"Later in the season, let's see how things will progress, let's see what objectives individual drivers can maybe achieve and if we need to adopt a different strategy, but at the start of the season, they will be free to fight."