Like Campbell, the Leafs have given Matthews maintenance days throughout the season and who can blame them? After the Leafs played 4 games in 6 days, it shouldn't come as a surprise that he's been given an extra day off.
The other reason he could be absent from practice is that he is joining Dubas and company in meeting with Matthew Knies in the hopes they can convince him to sign with the Leafs for the remainder of the season instead of going back to the University in Minnesota for another season.
That last part is pure speculation, but there's a good chance that his presence in the meeting will undoubtedly play a large roll in having Knies sign with Toronto.