Al has never been a good shotblocker but he doesnt have to do too much of it in this system. Its more based on position defense as well with team defense. He can learn that on the fly, hes not a rookie and has been around long enough to understand what good position defense requires.
Tim can take care of the main shotblocking for the most part, but Jefferson would bring some MUCH needed help for Tim in the low post scoring area. I can imagine the high-low scoring combos they could create for each other on a nightly basis.