The 19-year-old has had to bide his time at Elland Road through a combination of injury and the form of Kemar Roofe.
But with top-scorer Roofe and summer signing Patrick Bamford both sidelined, Roberts has taken his opportunity with both hands.
The Wales international made a promising first Championship start in the draw at Millwall and opened his Leeds account in style against struggling Preston.
After captain Liam Cooper gave Leeds a deserved first-half lead, Roberts netted his first United goal with a brilliant lob and headed home a second late on.
“He started the pre-season work later than others,” said Bielsa of Roberts.
“He has done many efforts. I asked him to play as a right winger and heard he liked to play as a number nine. Circumstances offered him a place in the team in the position where I thought he didn’t have to play.
“In the game against Millwall he had four chances to score and today scored two goals so it’s an interesting performance.
“He’s a very young player and I’m very happy for him. He deserves it.”
The result stretched Leeds’ unbeaten start to the Championship season to eight games and kept Bielsa’s side at the summit.
The Argentinean hailed a complete team performance, saying: “The first half was very good. The second half at the beginning was harder then we dominated.
“Individual performances were important and our offensive game and defensive game were good. There are few things to criticise in this game.”
“Our team was very dynamic and what we tried to do made us make links between the movement of players. Players were moving to create space for others and this worked pretty well.
“We have to underline the good performances of the individuals. Each one was high.”