The 32-year-old is now free to talk to foreign clubs and negotiate a pre-contract agreement, and he will be leaving Spurs on a free transfer at the end of the season if no extension is agreed.
Tottenham are expected to exercise their option to keep Son at the club until 2026, but it is not clear whether that could precede a long-term renewal for the South Korean, who turns 33 next year.
However, former Tottenham number one Keller has no doubts that Son should remain at the Tottenham Hotspur Stadium, highlighting his on-field feats and also his standing with the club's fanbase.
"Son has been a brilliant player for Spurs," Keller told Sports Mole. "You see he plays football with a smile on his face, he has good strengths and attributes all strikers need.
Keller confident Son will stay at Tottenham
"It would be sad to see him leave and I expect him to stay at Spurs. He can help the youngsters and also contribute as the leader in the team. You can see as well he is loved by the fans and gets respect from the players too."
Son is not believed to have given any indication that he wants to leave Tottenham in the foreseeable future, nor have there been any credible transfer links with particular clubs despite his contract uncertainty.
However, if the South Korea international is not offered a new deal beyond 2026, the summer window or next January would be Spurs' only chances to recoup a portion of the £24.9m they paid Bayer Leverkusen for his services in 2019.
Approaching his 10-year anniversary as a Tottenham player, Son has scored the fifth-most goals in Spurs' history with 169 in all competitions, only behind Harry Kane (280), Jimmy Greaves (268), Bobby Smith (208) and Martin Chivers (174).
The 32-year-old has amassed seven goals and six assists from 22 matches in all competitions during the current campaign, but he has failed to directly contribute to a goal in any of his last three Premier League games.
Should Tottenham keep or sell Son?
Held in extremely high regard by Ange Postecoglou as Tottenham's captain, Son still has a decent amount to offer in a Spurs jersey, as evidenced by his one goal and two assists in December's 5-0 battering of Southampton.
However, the South Korea skipper has appeared fatigued in recent weeks and was taken off just past the hour mark in the weekend's 2-2 draw with Wolverhampton Wanderers, perhaps a sign that he is not as young as he once was.
A 12-month extension certainly makes sense for all parties, as it would either allow Tottenham to postpone their search for a successor for another year or garner a nominal fee through a summer sale, but it is difficult to envisage a new long-term deal at his age.
Furthermore, Postecoglou has one of the most exciting prodigies in the Premier League ready to fill his boots in Mikey Moore, who is already making waves in the senior group at 17 years old and should play an even more prominent first-team role next season.
There are many 'what ifs' to Son's situation, none more so than whether the 32-year-old can continue to deliver in the 2025-26 campaign, but leaving next year on the brink of his 34th birthday may mark the natural end of an era.
Kasey Keller was speaking to Sports Mole on behalf of William Hill Vegas.
Written by
Ben Knapton