Mattock has been backed by Dan Ashworth to learn his lesson from his recent court case and get his career back on track.
The Baggies technical chief told the club's website: "It was a worrying time for Joe, for his family and everyone involved.
"People forget Joe has only just turned 20.
"At that age you're not quite as worldly-wise as you should be.
"I'm sure Joe will have learnt from the error of his ways, will knuckle down and continue what is a very, very promising career.
"He's still a young boy and he's achieved a lot in his short career.
"He played a lot of first-team games at Leicester and he played a lot for us last season.
"He got to the European Championship semi-finals with England Under-19s."