The talented defender suffered serious ankle ligament damage last season.
The injury was sustained during United’s quarter-final clash with Anderlecht in the Europa League.
He is not expected to be back in action until September at the earliest.
Herrera met up with Rojo in Portugal as the Argentine looks to bounce back from his injury hell.
Herrera posted a picture on Instagram with the caption "I am so happy to see that your recovery is going well, hopefully this visit gives you even more energy."
United’s stars report to pre-season training on July 8.
They will then fly out to the USA and face LA Galaxy in Los Angeles on July 15.
Their trip Stateside concludes with a clash against La Liga heavyweights Barcelona 11 days later.
United will also face Real Salt Lake, Manchester City and Real Madrid in between.
United chief Jose Mourinho won’t want Herrera mingling with Barca bigwigs during their trip across the pond.
The midfielder had been linked with a move to Catalonia.
But Spanish outlet Diario Gol claim Mourinho has informed Barca that Herrera will not be signing for them this summer.
The United boss has apparently labelled the club ‘a boat going adrift’ compared to the team he is building at Old Trafford.
