The 22-year-old has broken through at the La Liga side over the past 18-months and caught the eye this season, scoring three goals and providing four assists in 24 league games.
Betis face a fight to hold onto the Dominican Republic-born Spain U21 international, though, as he has a €50m release clause in his contract that both United and Liverpool could trigger.
Ole Gunnar Solskjaer is on the lookout for young players this summer with 21-year-old Daniel James joining from Swansea and Aaron Wan-Bissaka, also 21, expected to move from Crystal Palace.
Jurgen Klopp, meanwhile, is in the market for a left-back after allowing Alberto Moreno to leave Liverpool at the end of his contract and Firpo’s attacking instincts would fit well with his team’s philosophy.