Janssen was overlooked for a starting berth, despite Harry Kane's injury, with Heung-min Son entrusted to lead the line at White Hart Lane.
But the Dutchman was called upon for the final five minutes of the game as Spurs battled to keep an admirable Saints fightback at bay.
Janssen produced a misplaced pass with his first touch of the ball but recovered to ensure Southampton's defenders were afforded precious little time on the ball as they attempted to find a route to goal.
And Pochettino was delighted with the enthusiasm Janssen, who needed a "wake-up call" earlier in the season, showed on his 20th top-flight appearance for the north Londoners.
"It was good," Pochettino said of Janssen's cameo appearance. "He will have the possibility to play more and be more involved.
"I think today he showed great energy when he was on the pitch."
Meanwhile, Christian Eriksen and Dele Alli's goals had appeared to put Spurs en route to a straightforward victory.
🗣 "I don't think it was a brilliant performance but it was a solid one." 🗣— Tottenham Hotspur (@SpursOfficial) March 19, 2017
Mauricio on a 2-1 win v Southampton. pic.twitter.com/OABdRSGD8t
But James Ward-Prowse's effort gave Southampton a lifeline and the hosts were forced to endure a nervous finale before eventually cutting Chelsea's lead at the top of the table to 10 points.
Pochettino's side faced a similar scenario in their 2-1 win over Everton and the Argentine coach is eager for his players to kill games when they are in the ascendancy.
"We showed a lot of positives. It is in our nature to find a negative.
"We need to improve and score a third or fourth [goal] and finish the game because we allowed them to believe," he added.