The 34-year-old flanker, who won two Lion caps in New Zealand after joining the tour as a late replacement, will work on the contact area and the lineout at the Parc-y-Scarlets under the guidance of head coach and ex-Wales international Nigel Davies.
"I have no doubt Simon can be an excellent coach," Davies told walesonline.
"He's started well, and has the respect of the players - they hang on his every word."
Despite being born in England and earning 65 caps for Ireland, Easterby has been involved with Welsh rugby for a decade, first with Llanelli and then with the Scarlets when regional rugby was introduced in 2003.
And, despite warning Easterby that the road into coaching will not be a simple one, Davies believes West Wales' favourite adopted son will be hugely successful in the long run.
"It's not easy - I was a player-coach myself - but Simon knows that and he's equipped himself very well," added Davies.
"It's a difficult transition, but Simon still has plenty to offer on the field, and he has shown a lot of promise as a coach."