Lamela completed a £30m move to Spurs three days before the transfer window closed, with Andre Villas-Boas smashing Tottenham's transfer record to land the winger.
Sabatini has revealed that Lamela received an offer from an unnamed Italian club during the close season, which prompted Roma to start planning for life without the Argentine international because they could not compete financially.
He is quoted by Football Italia as saying: "It was a painful sale for all of us. At the beginning we didn't think we could sell Erik and we didn't want to. Then some new factors came into play and as time went on we started to consider the eventuality.
"A club made him an offer that we didn't and couldn't match. When that Italian club offered him €3.5m a season, as well as a €2m commission to his agent father, we knew we had lost the player.
"His people started pushing for a renewal and that is when I started moves to sell him. Had we begun negotiations sooner then we probably could have sold him for an even bigger fee. Roma are not a selling club. We are working with the objective of being competitive."
The 21-year-old is in contention to make his home debut for Spurs tomorrow afternoon, with Norwich City the visitors to White Hart Lane.