While I agree that Matic has actually played alright when called upon recently, he had made it clear he didnt want to stay around given he was behind McTominay, Pogba and Fred in the pecking order. Therefore the other three fit senior players who better fit the direction of the club, play.
You don't put him in the squad as if he's an equal part of it, then treat him like he's not an equal part of it, and expect the fans to figure that out and understand. That makes zero sense.
Solsjkaer must simply believe the others are better starting options than Matic at the moment, rightly or wrongly.