JCR reports the following:

According to a source, minor league hitting coordinator John Mallee is a finalist to replace Rudy Jaramillo as the Texas Rangers hitting coach. Mallee is believed to be one of six in the running for the job along with former Rangers outfielder Rusty Greer. The Dallas Morning News also lists Thad Bosley, Gerald Perry, Carney Lansford, Rick Down and Scott Coolbaugh as possible candidates.

Just read the whole story to get the possible impact.  Personally, I don't think it is that dire, but I don't like losing instructors, since it could cause a negative impact.  However, most of the players mentioned can hit, and it wouldn't matter who the coach was.  That said, a good hitting instructor in the minors can help a marginal player get better while at the same time help a talent to progress.

From what little I know about the Texas Rangers, unless Mallee is willing to take rock bottom dollars, I doubt there is much to be concerned about.