LAKELAND, Fla. (AP) If the question about Miguel Cabrera is how much longer he can be great, then the question about J.D. Martinez is even more tantalizing:

How great can he become?

The 28-year-old Martinez enters his third season with the Detroit Tigers after emerging as one of the American League's top power threats. Since being released by Houston in March of 2014, Martinez has been one of baseball's biggest surprises, hitting 61 home runs over the past two years. With some of Detroit's biggest stars now showing signs of age, Martinez is a major reason the Tigers can still hope to contend in the AL Central.

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And he still has room to improve.

''I definitely feel that my mental side of the game could be stronger. The difference in my eyes between superstar players and All-Star players is right here,'' Martinez said, pointing to his head. ''When you watch Miggy, and the way he handles his business, he looks like he's joking and he's playing around all the time, but the moment he gets in that box, he's a different animal.''

When the Tigers signed Martinez to a minor league deal in 2014, they could hardly imagine what an important addition he would become. He was called up to the majors about a month later and hit 23 home runs in 123 games, helping Detroit to a fourth straight division title.

Last year, the Tigers sank to last place, but Martinez showed his power was no fluke, going deep 38 times and driving in 102 runs. He attributed his improvement to a swing overhaul in which he altered his leg kick and the placement of his hands.

Those adjustments suggest Martinez already has a solid mental approach to the game, and manager Brad Ausmus noticed significant improvements last year in right field.

''He made huge strides,'' Ausmus said. ''Going back on balls, reads off the bat, he threw very well. ... He put a lot of hard work in.''

The 6-foot-3, 221-pound Martinez is not a huge asset on the basepaths. He was an All-Star last year, but he also led the Tigers in strikeouts with 178, while drawing only 53 walks.

But his power now appears comparable to the 32-year-old Cabrera's, and at this point Martinez looks like less of an injury risk. This offseason, the Tigers and Martinez agreed on an $18.5 million, two-year contract that took care of the outfielder's last two years of arbitration eligibility. He can become a free agent after the 2017 season.

At spring training Saturday, Martinez described contract talks as a ''negotiating battle'' but said it's part of the game.

''I wouldn't have signed it if I didn't think it was a fair deal,'' he said. ''I didn't want to start off the year on a negative note, going to arbitration.''

Martinez and the Tigers didn't agree on a longer deal, so Detroit might lose him in a couple years, but for now, the Tigers have a slugger in his late 20s for a price well below what they're paying some of their other standouts.

Martinez isn't taking his success for granted. He said he gets antsy if he goes too long without doing something baseball related, so even during the offseason, he was back swinging the bat in November.

''This game is weird in a sense that, the moment you think you've got it figured out, it'll humble you real quick. It'll step on you,'' he said. ''I'm a worker. That's always how I've been. I'll never stop, because it's just, it's in my blood. If I'm not doing something, I feel like I'm wasting my life.''

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Follow Noah Trister at www.Twitter.com/noahtrister