Both players were hurt in Thursday’s 2-2 draw against Sevilla in the Europa League quarter-final first leg at Old Trafford, in which United conceded two late own goals.
Martinez, who won the World Cup with Argentina in December, pulled up in pain gripping his right ankle late in the match and was taken away on a stretcher.
United had already lost fellow first-choice centre-back Varane to injury at half-time and his replacement Harry Maguire scored an own goal in stoppage time.
Manager Erik ten Hag was unable to provide an update when he faced the media at lunchtime on Friday but the club later confirmed the severity the injuries.
“Lisandro Martinez has been ruled out for the remainder of the season after fracturing a metatarsal bone in his foot,” United said in a statement.
“However, the Argentinian defender is expected to make a full recovery in time to be ready for the start of next season.”
The statement added: “Lisandro’s defensive partner Raphael Varane was also substituted with an injury at half-time in the same game and is expected to be out for a few weeks.”
Martinez — in his first season at Old Trafford — and Varane have established themselves as Ten Hag’s preferred centre-back pairing and their absence leaves a void as United look to end the campaign on a high.
Injury crisis
Ten Hag’s side, who won the League Cup in February, are looking to qualify for the Champions League and enter the weekend fourth in the Premier League standings.
Sunday’s trip to relegation-threatened Nottingham Forest is followed by the second leg of United’s Europa League quarter-final against Sevilla in Spain.
Bruno Fernandes misses that match through suspension and club captain Harry Maguire is then banned for next weekend’s FA Cup semi-final against Brighton at Wembley.
Maguire’s absence is a concern as he and Victor Lindelof are now the only out-and-out centre-backs at United’s disposal.
Asked how much faith he had in the pair, Ten Hag said: “A lot. I think we have decent centre-halves and they have proved in the past. They have proved this season.
“We have four or five very good centre-halves who can do the job.”
Another option is left-back Luke Shaw, who has impressed when used centrally this season.
The England international has missed recent matches through injury but could return against Forest along with midfielder Scott McTominay.
Top scorer Marcus Rashford is out with an apparent groin injury, joining Alejandro Garnacho and Donny van de Beek on the sidelines.
Ten Hag underlined the importance of squad players stepping up at his pre-match press conference.
“Every game is a battle, every game is a fight and you have to be ready for that fight,” he said.
“We have a squad. I have said all season we have more than 11 starters. So now others are on and when their time is there you have to show and contribute to the team.”
AFP
