Mattis told BBC Radio Sheffield: "I spoke to John Sheridan and he was very positive in the way he spoke about the club and where he wants to go."
"That's one of the main reasons why I decided to drop down from League One to League Two."
Mattis began his career as a trainee at Huddersfield in 1998 before moving to Bury in 2004 and then Barnsley in 2007.
After a month-long loan at Walsall in September 2007 he made a permanent switch to the Banks's Stadium the following summer after which he made 78 appearances for the Saddlers.
Source: BBC Sport
Source: BBC Sport