It could be argued that Liverpool, without a title in 27 years, are in a similar position, though the key difference is that European football is something of a passion on Merseyside, and, despite the domestic drought, the club saw Champions League success as recently as 2005.
"He's a very important player for me - much more important than you can imagine", Mourinho said on Monday.
The Reds begin their Champions League campaign with a home match against La Liga side Sevilla whereas the West London club welcome Azerbaijan outfit Qarabag to Stamford Bridge in midweek. There are a lot of games scheduled for this month and we will need the help of every single player in order to win as many games as possible.
Spurs have undoubtedly been dealt a tough hand in Group H as they were drawn into the same group as reigning champions, Real Madrid, as well as APOEL Nicosia and the aforementioned Borussia Dortmund.
Keeper De Gea, 26, said: "A club like Manchester United not to be in those games, it was hard to watch - but now we are back". Their last visit to Old Trafford in 2011 yielded a 3-3 draw and the reverse home fixture ended in a 2-1 win for the Swiss side.
And they will debut this Tuesday against FC Basel at Old Trafford, one of the most magical venues when it comes to European miracles. Spartak, which won the Russian Premier League, is in the group stage for the first time since 2012-13, while Slovenian team Maribor is the big outsider in its third attempt to reach the knockout stage.
United competed in the Champions League for 18 consecutive seasons before only qualifying for it in one of the last three and Mourinho admitted this will be a new experience for some of his charges.
- CSKA are making their 11th Champions League appearance - their fifth in a row.
But manager Mourinho is adamant it would be a huge blow if the former Everton player fails to play tonight - though his taking part in training yesterday seemed to at least give him a chance.
In total, United have played four times against Basel and all the previous meetings have come in the Champions League.
Now, Basel's best offseason signing is a player who already failed to meet the Premier League's standards. It's more comparable to the style of the Portuguese league.
The 20-year-old must first take on the challenge of replacing Neymar at the Nou Camp, following the Brazilian's world-record €222m move to PSG, before setting his sights on dominating the Champions League stage. To reach the semi-finals would be an achievement, the quarters par for the course.
Mourinho is one of five managers to have won the European Cup/Champions League with two different clubs, alongside Ernst Happel, Ottmar Hitzfeld, Jupp Heynckes and Carlo Ancelotti.