The 19-year-old striker is no stranger to the first-team fold at Anfield.
He has previously figured in pre-season fixtures and been named in matchday squads by Jurgen Klopp.
A youngster who helped to fire England to U17 World Cup glory in 2017 is, however, still waiting on competitive minutes.
An unfortunate injury has stunted his progress this season, but his manager has “guaranteed” that he will be involved in a semi-final clash with Barca that Mohamed Salah and Roberto Firmino are set to sit out.
The teenager is expected to slot seamlessly into the fold, with Robertson telling Liverpool’s official website: “Rhian's a different kettle of fish. He's been out most of the season. Now he's come back and he's been doing well in training and scoring goals for the U23s.
“If he is involved or whatever then it'll be incredible for him. He's a confident lad and he's got no doubt in his ability and neither does anyone at Melwood.
“When he gets his chance, it's about showing it to everyone else because we know how good a player he is. Let's see if he gets his chance. But, if not, then his chance is coming sooner rather than later.”
It is unlikely that Brewster’s big break will come from the start against Barcelona, despite Salah and Firmino nursing knocks.
The likes of Xherdan Shaqiri, Divock Origi and Daniel Sturridge are pushing for places in Klopp’s XI, with Robertson hailing the depth of Liverpool’s squad at a testing time.
He said of those on the fringes of a fold pushing for Premier League and Champions League honours: “They've been fit and carried this squad obviously with the disappointment of not playing.
"But they've used it in the right way by putting 100 per cent in training and pushing the lads that are playing to their absolute maximum.
“That's why I think the boys on the pitch have played so well this season - because we get pushed by the people off it. But people on the outside don't really see that and the lads won't get the credit they deserve, but the whole squad have been magnificent.”
Liverpool need somebody to offer inspiration from somewhere against Barcelona, with the Reds trailing a heavyweight last-four encounter 3-0 heading into a must-win fixture at Anfield.