The Giants will definitely add a third quarterback, but not this weekend.

They will wait and watch to see how Jameis Winston performs Sunday as he starts in place of out-for-the-season Jaxson Dart against the Titans at MetLife Stadium.

And then, on Monday, the front office will meet and figure out the best course of action, The Post has learned.

This suddenly dire situation means the entire Giants’ offense is one play away — one Winston injury — from having Jake Haener as the starting quarterback.

And that is far too much of an unknown for anyone to feel comfortable with so early in the season.

Winston was the emergency fill-in last Monday night at SoFi Stadium when Dart went down on the seventh offensive snap with a left knee injury.

Dart had surgery Friday to repair the meniscus and medial collateral ligament damage in the knee, with a recovery time estimated at four to six months.

Winston, the 32-year-old veteran, came off the bench and played poorly, skipping and sailing throws, completing only 11 of his 27 passes for 111 yards and one terrible interception in the end zone.

His passer rating was 37.7.

The Giants hope with a full week of practice with the starters and with a game plan suited to his skill set, Winston will play much better when he faces the Titans in what will be rainy and windy conditions.

Winston started two games last season when Dart was in concussion protocol, and the Giants led in the fourth quarter in both of those games, a 27-20 loss to the Packers and a 34-27 overtime loss to the Lions.

He was given a two-year, $13 million contract extension on Sept. 6.

Haener, 27, was a 2023 fourth-round draft pick of the Saints out of Fresno State. He has one career NFL start.

The Giants plan to add an experienced quarterback who can come in, learn the system and eventually provide more security at the position than Haener can provide.

They prefer not to give up much draft equity, but they realize trading for a quarterback might be their best option.

Backups around the league include Mac Jones, J.J. McCarthy, Justin Fields and Gardner Minshew. Jimmy Garoppolo was the Rams’ backup the past two years but is not currently in the league.