Fabregas, whose move to the European champions was worth a reported £35million, was also on target in their 2-0 UEFA Super Cup victory over Porto.
Barca chief Bartomeu says after seeing the 24-year-old in action on the pitch and in training he believes he is worth closer to the €60m (£53m) price tag originally bestowed on him by the Gunners.
He told Sport: "After seeing the work he's done since his arrival it is true that Cesc meets the €60m valuation Arsenal initially demanded.
"The kid is demonstrating a high level and quality in three weeks of training. The transfer was very good.
"In a market that is so open Cesc is worth a lot of money.
"It was complicated to close the deal. The negotiations were very hard."