Ex-Reds defender and academy coach Tonge, 33, was recently brought in to assist Stendel and existing deputy Christopher Stern for an initial three-game basis following Andreas Winkler’s departure.
After impressing, Tonge will remain in situ for the rest of the season and possibly beyond.
Stendel said: “We said two weeks ago that after the three games we would see at that moment and then in the summer what the next step would be.
“All things are possible and at the moment I am happy I have Chris and Tongey.
“In Germany we (would) say that he (Tonge) speaks the language of a player.”
Tonge, part of Barnsley’s promotion class of 2005-06, said of his appointment: “It was as big a privilege to be asked to walk out to the dugout as it was to putting on the (first-team) shirt at 17.
“It is exciting, but not something I am fazed by.”
Meanwhile, Reds chairman Paul Conway has hailed the club’s business in the recent January transfer window as a success.
It saw top target George Miller brought in from Middlesbrough even though the lack of a recall clause in his loan deal at Bradford City means that he will not don a Barnsley jersey until next season.
With one eye on balancing the budget after the estimated loss of £6m in revenue following relegation, the sales of Brad Potts and George Moncur were sanctioned, but interest in the likes of Ethan Pinnock, Kieffer Moore and Liam Lindsay was significantly repelled.
Conway, who has assumed CEO duties for an interim spell after Gauthier Ganaye’s exit to Nice, said: “We believe we had a very strong window. George Miller was our top target and we believe he is a top-10 League One player. Jordan Green was also an important addition that the coach wanted.
“We are commited to keeping a good squad together and throughout the summer and winter our goal was very clear with automatic promotion. Of course there was a lot of interest in the squad, (but) we basically had a not for sale sign out and everything was pointed to automatic promotion.”
Young striker Dylan Mottley-Henry has joined Harrogate Town on loan.