By Bruce Levine-

MESA, Ariz. (CBS) — After two of the worst seasons a Cubs starting pitcher has ever produced, right-hander Edwin Jackson has challenged himself to be better.

READ MORE: 5 Wounded In South Austin Mass Shooting

Jackson’s resume has taken a huge beating over the past two seasons, during which he’s gone a combined 14-33 with a 5.58 ERA that would send most players into another profession. For the 31-year-old Jackson, it pushed him into the weight room.

To his credit, Jackson put in the extra work in the offseason that hopefully will pay dividends for him and the team in 2015. Jackson was 6-15 with a 6.33 ERA in 2014.

“This was a pretty busy offseason for me,” Jackson said Thursday. “My off time was cut short this fall. It wasn’t like I was out of shape, but I got into the gym and started early to get in better shape. In my mind, that was the start of the mental challenge of the game. First push myself in those workouts, then concentrating on the mental strength. I believe the two areas go together — mind and body as one .”

Having pitched in eight different organizations, Jackson has as many friends in the game as any current major league player, and few people in the game are as well-liked as the Cubs pitcher. That aside, like any business, results are what matter when the day is complete.

“It got frustrating for sure,” Jackson said. “As a professional, you must keep the same pace and stay positive.”

Cubs fans have been tough on Jackson, and rightfully so. When he signed a four-year, $52-million deal before the 2013 season, they were told he was coming to Chicago with a resume for being an innings eater. Other attributes Jackson was known for was being a leader of a young staff and well-liked individual.

READ MORE: 15-Year-Old Among 2 Shot In West Englewood

Only the nice-guy part has shown up so far. Cubs manager Joe Maddon is familiar with Jackson, having managed him in Tampa Bay in 2007 and 2008.

“His clock should not have been pushed so quickly so early,” Maddon said about Jackson’s quick ascension through the Dodgers’ system. “I just don’t think he had enough time to learn his craft. The last couple of years, I don’t know because I have not seen him. He is a horse and gets better the more pitches he throws and the deeper in the game he gets.”

The problem is that Jackson has rarely gotten past the fifth inning with the Cubs. He has averaged just 5.4 innings per start in a Chicago uniform.

“It is rare that a guy gets to 30 years of age and still has the label of potential attached to his ability,” Maddon said. “It all has not arrived with him yet. That is really interesting with a guy like him. There is room for improvement. He knows that is true. When we had him with the Rays, there was that moment that he couldn’t move past the first three or four innings that would beat him up. When he got through that, he got better as the game was going on.”

Jackson’s $52-million contract still has $22 million left on it over the next two seasons. He hears about his past failures regularly when he receives fan mail. Taking the critics in stride is part of the gig for Jackson.

“If you believe what everybody says about you, eventually you will be sitting and watching with them,” Jackson said. “In sports, everyone has an opinion. You can be the best player in the world, and someone will hate you. What matters is what you think and how you approach your job.”

MORE NEWS: Man's Body Pulled From Lake Michigan In Evanston, Hours After Crews Rescuing 3 Other People Find His Belongings On Beach

Bruce Levine covers the Cubs and White Sox for 670 The Score and CBSChicago.com. Follow him on Twitter @MLBBruceLevine.