In the two seasons that followed his 2011 arrival at Stamford Bridge from Valencia, the Spaniard has contributed 32 goals and 55 assists in a blue shirt.
When asked about the situation, Cascarino, now a football analyst for The Times, believes that the 25-year-old is more than worthy of a place in Mourinho's team.
"It's all to do with the formation. If they go for a five-man midfield, Mata will be included. He is not someone that hugs the line, he comes inside and looks for opportunities. He can also come up with a goal," he said.
"Mata has really had his nose put out of joint. He is one of the big favourites of the fans and they are a bit baffled [as to why he isn't in the team regularly], but most of them are giving Mourinho the benefit of the doubt. It doesn't look like he's going to play that regular, it just depends on the opposition.
"It's very hard to ignore him. I love the way that he interlinks with Oscar and Eden Hazard. He is always looking to be creative. How can you leave out a player that can score around 20 goals a season? I find it very hard to understand. I can see why David Luiz has been left out, but it's hard to come to terms with Mata not being in the team. He's such a quality player that he'd get in at any of the other leading sides in the Premier League."
Mata has scored one goal from his 12 outings in all competitions so far this season.
Watch Tony Cascarino's Monday Inquest on The Times Goals, which brings subscribers Premier League goals and highlights before anyone else on mobile, tablet and online. Visit www.TheTimes.co.uk/football to find out more.