Kane limped off during the 1-0 loss at St Mary’s and Tottenham manager Jose Mourinho admitted he feared it would be “bad news” once the England captain had scans on the injury.
Those fears have come true, with Tottenham saying in a statement: “Following assessment today (Friday), we can confirm that Harry Kane suffered a tear in his left hamstring during our New Year’s Day fixture against Southampton.
“Our medical staff will continue to review the injury with treatment ongoing.”
Kane responded to the blow by tweeting: “Head up. Tough times don’t last, tough people do.”
Mourinho had already ruled Kane out of Sunday’s FA Cup third round trip to Middlesbrough, but he expects the club’s leading scorer to be out for much longer than that.
Tottenham have yet to give a timeframe for Kane’s return.
“I don’t know yet, maybe later today we have news but if you ask me just my feeling, good news or bad news, I am more on bad news than good news, that’s my feeling,” Mourinho said earlier on Friday.
“What the player felt, Harry Kane leaving a match, the way he did it, he didn’t think twice, it didn’t take him two seconds to realise the severity of the situation.
“Some opinions from the medical department before the test results. I don’t think we’re going to have good news.”
The loss of Kane is a huge setback for Spurs, who are stuttering under Mourinho after an initial bounce following his appointment in November.
Kane has scored 27 goals in 31 appearances for club and country this season.
“Everybody knows who he is, what he is, what he means for the team, the fans, the club,” Mourinho said.
“His quality, the routines that the team has playing with him, every minute of every game he doesn’t play we miss (him), so every match that he doesn’t play we’re going to miss him.”
Christian Eriksen will be at the Riverside Stadium even though his future is unclear.
The midfielder is now free to discuss a move with European clubs while he might also be sold, after telling Spurs he will not be signing a new contract.
“I don’t know and I am being honest,” Mourinho said when asked about Eriksen’s future. “You know the situation. In this moment he can agree with any club for the end of the season. That is one option.
“Another option is a club and himself want to do a deal in January, but in that situation our club is in control.
“We have the control of the situation. Another possibility is he stays. In this moment he is here, he is our player.”