When his chances were curtailed by the Portuguese boss, Rashford secured a dream switch to the Camp Nou and has shone during his stint with the Catalans. While his side suffered a 2-0 defeat at home in the first leg of their Champions League quarter-final clash against Atletico Madrid, Rashford was outstanding, finding the net only to be narrowly ruled offside. He was Hansi Flick's side's most threatening player, registering four shots on target, more than any other player on the pitch.
Yet the most notable transformation Warnock identified was the winger's willingness to take on opposing full-backs and beat them at will, which may go some considerable way to persuading the club to sign him from United on a permanent basis.
Warnock, speaking on the BBC Champions League highlights programme, said: "He's just a brilliant player isn't he? I think what he's done since he's gone to Barcelona is, he's found his form. I think he's found a happy place where he's enjoying his football. I actually think he should do better in this position when it comes to him.
"His touch just gets caught under him ever so slightly. The strike isn't quite as clean but his movement is brilliant at the moment. What you want from a winger, on the opposing side, is to get in the box and get opportunities. Maybe he can take that down, but he's so confident in himself that he wants to take that shot on first time.
"Again, arriving at the back post, great goal,[later ruled out] and that's exactly what you're after. Hansi Flick will be delighted with that. When you're a full-back, and you're playing against someone who has pace, who can time his runs well and know he can use that pace, he's so dangerous in behind. Maybe he could show a little bit more composure, but that confidence he's got in himself, he's taking everything on.
"He's playing alongside (Lamine) Yamal, and you're thinking, he's the one over free-kicks, he's the one trying to be in the ascendancy, on the front foot, taking people on. I think you can see a huge change in character, in personality, in confidence from an outstanding footballer.
"I think from Thomas Tuchel's point of view, I think it's the best thing that's happened for him this year. It gives him another option in forward positions going into the World Cup."
Given his impressive performance in Catalunya, Rashford will be amongst his team's greatest assets when it comes to clawing back the two-goal deficit.
Whether Barcelona ultimately decide to exercise their option to purchase him permanently from United at the end of the season also remains unclear.
The Spanish giants have until 15th June to trigger a release clause in his contract that would allow them to secure his signature for approximately £26m from United.