Howells saw his weakened side beaten 48-0 by Cardiff Blues in their last outing 13 days ago.
It was their heaviest defeat of the season, however Dewey and Di Rollo missed the game with abdominal and neck injuries, respectively.
Webster has been out for two months with a left leg injury but returns to face the team who lost to Leicester in last weekend's Heineken Cup semi-final and were then beaten by Ospreys on Tuesday in the Magners League.
Webster features on the left wing for the away match, with Dewey named on the right and Di Rollo at outside centre.
Fly-half Duncan Hodge is promoted from the bench, with Ben Cairns switching to full-back as Alistair Warnock misses out.
Alasdair Strokosch and Fergus Pringle replace Scott Murray and Chris Bentley in the second row; Murray is injured, and Bentley starts on the bench.
Blind-side flanker Simon Taylor comes into the side at the expense of Alan McDonald.
Howells said: "I am pleased with the way the players have responded in training since the defeat in Cardiff two weeks ago.
"We realise we could face a backlash from the Scarlets given their loss earlier in the week and in the Heineken Cup.
"But we cannot worry about them, we have to concentrate on what we're doing and if we play to the best of our abilities, we're in with a shout."