Pochettino has doubts over the 32-year-old, with his performances so far this season not at the previous high level most have associated the Belgium international with since he joined the club seven years ago.
He was left out of the Spurs starting lineup for the first three Premier League games of the season, with Davinson Sanchez selected to start alongside Toby Alderweireld at the heart of the defence.
However, issues at right-back, with Kyle Walker-Peters injured and Serge Aurier out of favour at times, has meant Sanchez has filled the void at full back, allowing Vertonghen back into the team for the last four games.
Yet Vertonghen’s performances, along with the whole Spurs backline, have been sub-par, with the side conceding twice in three of their last four games.
According to Football Insider, Pochettino and his coaching staff fear his best days could be behind him after worrying signs in recent weeks. With his contract expiring at the end of this season, Vertonghen, who has played 212 times in the Premier League and was an integral part of the side that reached the Champions League final, could be set for a summer departure if his form does not improve drastically.
It seems the defender, who has also represented Belgium 116 times, must impress both the coaching staff and others in the club's hierarchy in the coming months if he is to earn himself an extension on his current deal.