Saints took on the Turkish side in the final friendly match of their trip to Austria and, despite falling behind to a wonder strike in the second half, they produced a superb late rally to turn the game around.
There was no place in the squad for either Maya Yoshida or Gastón Ramírez, who are both yet to see any pre-season action after being given some time off after the Confederations Cup.
The quaint surroundings of the Stadion Villach Lind were dominated by a loud band of Beşiktaş supporters, who made themselves well heard throughout the evening.
They nearly had reason to make even more noise in the tenth minute, when Manuel Fernandes’ free-kick was met by the head of Hugo Almeida, only for Artur Boruc to make an excellent save.
The only other effort of note in the first half came from James Ward-Prowse, whose free-kick on the edge of the area was tipped wide by Beşiktaş keeper Cenk Gönen.
Saints made three changes at the break, with Victor Wanyama, Lambert and Luke Shaw being introduced in place of Chambers, Guly and Danny Fox. Jack Cork switched to right-back, indicating that Pochettino views him as another potential back-up for Clyne.
It was Beşiktaş who started better, though, with one of their own substitutes, Dentinho, striking the crossbar with a side-footed effort from Gökhan Töre’s cutback.
Boruc appeared to get a hand to the ball, but a goal-kick was awarded.
The Saints stopper had to be alert again shortly afterwards to divert Almeida’s rasping shot wide.
But there was nothing he could do about the sublime effort from Muhammed Demirci in the 62nd minute.
The midfielder drove forward 30 yards out, shifted the ball onto his left foot and, from a central position, powered a shot past a full-stretch Boruc into the top corner.
Second-half sub Mayuka had the ball in the net ten minutes later, after Lambert’s clever pass slipped him in, but his low finish into the bottom corner was ruled out for offside.
The equaliser did arrive three minutes later, though.
Cork lofted in a free-kick from the right touchline, which Lambert was somehow allowed to chest down in the area. Allowing the Saints forward the opportunity to control the ball in such a fashion represented a dreadful piece of defending from Beşiktaş, and they were made to pay the full price, as Lambert lashed home with his right foot.
Saints completed the turnaround three minutes later, as Mayuka unleashed a dipping left-foot volley from the edge of the area that beat Gönen at his near post.
Isgrove then wrapped up the win on the stroke of 90 minutes, slotting in Shaw’s square pass from six yards with his left foot.
Saints will now head home for a few days, before departing for next weekend’s game against Celta Vigo, in Spain. Pre-season will then finish with the fixture against Real Sociedad on Saturday, August 10.
SAINTS: Boruc, Chambers (Wanyama HT), Fonte, Hooiveld (Lovren 70), Fox (Shaw HT), Cork, Schneiderlin (Davis 58), Ward-Prowse (Isgrove 70), Lallana (Puncheon 58), Do Prado (Lambert HT), Rodriguez (Mayuka 70).
Unused subs: Gazzaniga, Turnbull, Stephens.
BEŞIKTAŞ: Gönen, Kurtulus, Sivok, Franco, Gülüm (Süzen HT), Kavlak, Özyakup (Demirci HT), Fernandes, Töre (Toraman 73), Şahan (Dentinho HT), Almeida (Pektemek 63).
Unused subs: Güvenç, Akgün, Escudé, Uysal, Kayhan.