Write the letter by hand. Handwritten letters can imply more feeling and humanity in the words when written on the paper. Although it may take more work to push a pencil than it is to press keys on a keyboard, the letter's recipient might appreciate its non-sterile appearance, not to mention the thought and effort that went into composing each word.
Mention several reasons why the team member will be missed. Recognize their talents that made them such a valued team member. For example, they may be the fastest runner, best shooter or most clutch hitter on the team. Cite specific plays during games when they either made the winning score or directly helped the team win. Also mention several after-game get-togethers and how such gatherings won't be the same without their presence.
If they are ill and have to leave the team, include get-well messages wishing them a speedy recovery. Mention a personal anecdote of a loved one who overcame an illness. This can offer them a sense of hope knowing they are not the only one has dealt with the issue.
Deliver the letter with a gift. This could be a self-made scrap book of photos from games, practices and meetings, a bouquet of flowers or any other gift that you think might brighten their spirits. A signed game ball by every member on the team can be a great gift and preserve the memory of each team member.