Matty, 20, was called into the squad for only the second time at the weekend.
It was just reward after earning his first starts of the season and scoring his first ever Spurs goal in our last two games.
He has joined Fulham's Zat Knight, Newcastle's Jermaine Jenas and Charlton's Paul Konchesky, all untested at this level, in Platt's squad for the game in Nova Gorica (3pm).
Platt said: "This is an excellent opportunity to look at some new players, all of whom have done well this season. I am looking forward to seeing them in training and I’m sure they will integrate well into the squad.
"Obviously there have been a few changes in personnel. Wayne Bridge, Ledley King and Joe Cole have moved up to the senior squad for the Holland match, and they are there on merit.
"There are also a number of players — Jonathan Greening, Sean Davis, Luke Young — who I decided not to call up for this game as they have proven themselves consistently at this level and I know what they bring to the team.
"However, despite the absences, we will be travelling to Slovenia with a strong and experienced squad. "
The Under-21s have two further warm-up matches ahead of the European Under-21 Championship in Switzerland, both against top-class opposition.
They will play Italy at Bradford City on Tuesday March 26 and Portugal at Stoke City on Tuesday April 16.
England’s group opponents at the Championship, which takes place on 16th — 28th May, will be known at the draw at UEFA Headquarters in Nyon, Switzerland, on 25th February.
England Under-21 boss David Platt has hinted he will look at new faces in this afternoon's friendly against Slovenia - that could include Matthew Etherington.