The Spaniard joined Manchester United from Athletic Bilbao for £28.85m in 2014 but summer arrival of Fred, as well as Nemanja Matic's last summer, looked to have signalled the end of Herrera's time at the club but he is still an important player for Mourinho.
"So if you want to speak about their football potential we can speak, and football potential starts with investment".
When asked to elaborate, though, his response was unusually reserved, saying: "I keep for me".
Honing in on the acquisitions of Kyle Walker, Aymeric Laporte, Danilo and Benjamin Mendy, he couldn't resist having a dig at City's tendency to buy solutions rather than find them through clever management. "I don't think it's easy for him". "When I say fault, I mean for the right reasons", he said.
Mourinho himself has admitted to his players" inconsistency issues and believes it's a part of "growing up'.
United will be hoping for a repeat of last season's remarkable performance at the Etihad, which saw the visitors destroy City's dreams of winning the Premier League titles against their rivals.More news: Trump "insulted" by Macron's proposal for European army
How do I watch online live streaming of the Manchester City vs Manchester United Premier League match?
"I think the coach definitely finds it hard to put all of us in because we're playing really well and there's a really good level in attack".
"So if we keep conceding goals before the opponent we will arrive at the day where we can not come back so we have to make sure that we grow up, not just in our approach, but also in our compactness in order to start matches well".
The Manchester derby is once again upon us and, as always, this game is expected to light up the Premier League.
"If I need a full-back and I buy four, it is easier", Mourinho said, per the Sun.
"If the answer is positive and we have of course one more session tomorrow, if the answer is positive he is ready but he trains with the team I don't know his answer".