The Red Devils have invested heavily themselves in recent years, with Paul Pogba and Romelu Lukaku among those to have been acquired in big-money deals.
Others have adopted a similar philosophy, though, with the riches on offer in the Premier League allowing English sides to rebuild in each passing transfer window.
Mourinho admits his United side are struggling to overhaul the global elite as a result, telling reporters on the challenges he faces amid mounting criticism of his Old Trafford reign: “If the clubs that are in a better situation than us, if they stop investing in everything and we invest, I would say we are side to side.
“But if they are in a better situation than us, and they keep investing the same or more than us, it’s difficult. It's as simple as that – it’s difficult.
“I’m not saying [we will never catch up]. You have the best proof of it. I won the Champions League with Inter. I didn't win the Champions League with Real Madrid, or with Barcelona, or with [Cristiano] Ronaldo, or with [Lionel] Messi. When I was Ronaldo’s manager, we didn’t manage to win the Champions League.
“But we can [win it] and we are going to fight for it again, no doubt.”
United will have to wait at least another 12 months before conquering the continent as Mourinho has been seeking to justify his methods at United in the wake of a disappointing Champions League exit.
A 2-1 defeat to Sevilla ended the Red Devils’ involvement for 2017-18, with an ambitious side crashing out at the last-16 stage.
More funds are likely to be made available over the summer in an effort to go deeper into the tournament next season, with Mourinho expected to be back in the market for proven performers who are capable of helping United close the gap on domestic and European foes.