Henderson is believed to be a leading target for the Magpies, with the club seeking a new number one for the 2022-23 campaign.
The 25-year-old only made three appearances for the Red Devils last term, while he did not feature at all in the Premier League, with David de Gea the club's number one.
The England international was linked with a switch to Newcastle in January, but a deal could not be agreed between the two parties.
Darlow featured on eight occasions for the Magpies during the 2021-22 campaign, conceding 15 times, and he still has more than three years left to run on his contract at the club.
"After 511 appearances, I've decided it's the right time to move into the next stage of my career. There have been so many fun moments mixed in with the odd challenge, but I'm grateful for each and every one," Grant told Man United's official website upon announcing his retirement.
"Fulfilling a boyhood dream of playing for Manchester United was something I thought was out of reach, so to have achieved that fills me with great pride and emotion!
"Although I haven't played as many games in the last four years, I've given everything to the group every single day on the training pitch and in the dressing room. The opportunity to join the biggest club in the world was one I could never turn down and I'm so grateful for the lessons learned and the friends I've gained.
"The opportunity to challenge, set standards and ultimately learn from some truly exceptional players and staff has been so valuable."
Darlow has been with Newcastle since 2014, making 98 appearances in all competitions, including 26 outings during the 2020-21 Premier League campaign.
The 31-year-old played in eight straight Premier League matches for the Magpies between September 17 and November 20 before losing his position to Dubravka.
Man United are due to begin pre-season training at the end of June, although the players involved for their countries next month will be given additional time off.
The 20-time English champions will travel to Bangkok, Melbourne, Perth and Oslo to take on Liverpool, Melbourne Victory, Crystal Palace, Aston Villa and Atletico Madrid this summer as part of their preparations for the 2022-23 campaign. body check tags ::