Ole Gunnar Solskjaer has boosted morale since taking charge in December but some players have been surprised how difficult it has been to get their Three Amigos to commit, The Sun reports.
Three Lions defenders Ashley Young, Chris Smalling, Phil Jones and Luke Shaw agreed fresh deals this term but Spanish aces David De Gea and Juan Mata have yet to tie up extensions.
United have triggered a one-year option on De Gea’s contract, which now expires next season, but failed to match his market value of £350,000-a-week.
PSG are prepared to meet his demands and will bid this summer if there is no agreement or wait 12 months if United reject their approach.
The French giants have negotiated with Herrera and were willing to break through the £150,000-a-week barrier for the midfielder.
Mata is the other Spaniard in limbo and is assessing his options for next season as his United contract comes close to expiring.
New contracts have been easier to negotiate for Young, Smalling, Jones and Shaw.
Young, who will be 34 in the summer, has secured a one-year extension, while Shaw was out of the picture last season under Jose Mourinho before battling back.
Smalling and Jones, meanwhile, put an end to uncertainty over their futures by securing long-term deals to remain at the club.
France winger Anthony Martial also agreed a new contract, while Scotland midfielder Scott McTominay was the other first-team player to get fresh terms.
Ecuador full-back Antonio Valencia is expected to leave in the summer on a free transfer after 10 years at the club.