Some parts of his game will be difficult to replace. And yes, I mean replace, because until the Twins at least make an offer there's no reason to believe he'll be back.
Getting On-Base (Career OBP: .324)
Patience and taking walks have never been strong points in Hunter's game, and even though his aggressive nature at the plate can make him dangerous it also displays a lack of discipline. Torii walked in just over 7.5% of his plate appearances in 2007; that's not easy to do but it's probably easy to do better. This is why replacing Hunter's on-base skills will be one of the easiest, and most inexpensive, assets to find a new suitor for.
The Plate Appearance (Career P/PA: 3.51)
For a career, seeing three and a half pitches each time you come to the plate isn't horrible. It's not good but it could certainly be worse. But continuing the theme of looking behind the traditional statistical lines, Torii only saw 3.37 P/PA this past summer. This is fewer than any position player for the Twins receiving a plate appearance in 2007 not named Chris Heintz or Matthew LeCroy.
Hunter's second consecutive career year came in spite of his aggression at the plate being even more severe than it had been in years past. (His BABIP was up, his strikeout rates were down and he still saw the fewest pitches per plate appearance since becoming a full time player.) Attributes which allow a player to take more pitches, draw out at-bats, and make a pitcher show everything he has should also be inexpensive, and easy, assets to replace.
Power (2007: .218 ISO, 28 HR, .505 SLG)
This will probably be the most difficult, and most expensive, asset to make up--good isolated power, very solid raw power. In fact, even with the money available for the Twins to spend, to replace these numbers that Torii brought to the lineup Minnesota will have to turn toward players who aren't as complete as Hunter. In this category more than any other, the Twins may need to pay for something a bit more one-dimensional in order to stay within their team salary comfort level.
This doesn't mean go after a player like Tony Batista. It does mean that finding a player who can't catch, can't throw, can't steal bases, can't play defense, but CAN hit should be on the radar. Even if the player is aging, has some minor issues staying off the disabled list or even has some past dysfunction in his personal life, this kind of player still might be the best bet for the Twins. Power is an expensive commodity, and when Hunter goes, the Twins will need to find someone to offset the loss.
Defense (Career: 72 assists, 2.82 CF range factor, .896 CF zone rating)
Minnesota doesn't need to replace the history of Torii Hunter, which is the most difficult thing to wrap your mind around when thinking about the future of center field. Torii was great for a couple of years, and has been very, very good his entire career. Bill Smith just needs to replace who Hunter has been recently...which is no small task, either. In '07 he made plays on 47 balls out of his zone, made plays on 89% of the balls in his zone, and he still has a very strong and very accurate arm.
Realistically, unless the Twins somehow manage to land Andruw Jones, and they won't (80 plays made out of zone, made plays on 92% of balls in his zone), they won't be able to replicate Hunter's defense. Still, because defensive reputation can inflate a player's worth compared to how good he actually is, the cost of the defensive replacement for Hunter isn't an easy call. Sacrificing offense (or at the very least power) for a defensive-oriented player is the best route to take to ensure getting your money's worth on a player, since offense will not only inflate salary but generally his defensive reputation as well...which will inflate salary even more.
Speed and the Base Path (126 career stolen bases, 67.7% success rate, 82nd-84th percentile in raw speed)
Speed isn't as important as knowing how to use it: when Carl Crawford digs for third it's very realistic that the ball could beat him there. A big asset for Hunter in the field was his speed, where he was able to make up for slow jumps or difficult positioning to track down fly balls. On the bases Torii had developed a knack for making the most of his stolen base attempts by taking advantage of sleeping defenses, or by using what he knew about the catcher or the starting pitcher and his windup.
Inserting Alexi Casilla into the lineup replaces Hunter's raw speed (and more), but it doesn't replace his savvy for stealing bases or his speed in the field. Inserting Jason Tyner replaces Hunter's speed in center field and his raw speed, but it's unknown whether he still has what it takes to steal 20 bags a year at a 70% success rate. Speed on it's own is a fairly undervalued and inexpensive asset to purchase, which is promising, but for speed in the lineup there are two options already in house to replace the speed lost from Hunter.
Conclusions
If you buy into my evaluation, there are three parts of Torii's game that are fairly easy and inexpensive to replace: the ability to get on base, the ability to wear down a pitcher and speed. One metric I rated as difficult to judge but nearly impossible to replace (defense), and one metric I rated as expensive and difficult to reproduce (power). Those are a lot of things to find replacements for, but it's definitely easier (and cheaper) to find the pieces than to duplicate the entire package that Hunter brought to the table. Because the Twins are also auditioning for a number of starting positions (at least third base, center field and designated hitter), it means there are three starting positions open with which to put together a sum of parts to replace a whole. Or a hole. Both, really.
What kinds of players does this mean the Twins should be looking for? It's hard to say, but there are options. You could go for a speedy center fielder who can take pitches and hopefully have a stronger arm than Jason Tyner, or you could go with Mike Cameron who historically will give you some power and a better OBP than Hunter but is getting into the years where a player can suddenly just disappear. For power the Twins are probably best off looking to aging veterans like Mike Sweeney, Mike Piazza, Tony Clark or Preston Wilson; none of them are sure shots and all of them come with a gamble, but barring some unforseen Amazin' Trade the Twins won't get players the caliber of an Alex Rodriguez, Barry Bonds, Bobby Abreu or Mike Lowell.
Even if the Twins had managed to keep Hunter, there were going to be some gambles involved in filling out the position players on the 25-man roster for next year. With Torii's impending departure looming less than two weeks away there may be even more gambles, even though a couple of them could be marginalized by using the money that won't be used on our departing Gold Glover with the Silver Smile.