FC Barcelona fans are notoriously poor travellers for the group stages of the UCL. They presume - with some justification - that they will reach the latter stages of the tournament and save their money accordingly. The same applies to fans of Real Madrid. No doubt afew executive jets will turn up for 'corporate hospitality', but don't bank on many airliners beyond the team-plane. From an aircraft point of view, drawing Celtic makes it a double-disaster for MCFC with only BMG offering the prospect of a more interesting airlift.
The United matches offer better airlift potential. Dutch fans will travel, though from Rotterdam many may use the ferry. Likely to produce a couple of Transavias though. Fenerbahce ... we generally get a couple of passenger charters with Turkish fixtures. Zorya (Ukraine) are likely to produce an interesting team-plane, though not a large airlift.
From an aircraft point-of-view, the best scenario is to draw a minnow team from a wealthy smaller country where a trip to see their club play at the Etihad or OT is perceived as the opportunity of a lifetime ("the biggest match in club history"). If they can afford it, they'll travel. Scandinavia and Switzerland are good examples of this. So this year's draw hasn't been the kindest for aircraft enthusiasts. Zorya is the only 'minnow' amongst those six ties, and their home is in a war-zone. The match in Ukraine will be played in the West of the Ukraine, well away from the club's home city.
City and United fans can be expected to travel to the away fixtures in numbers as usual. However, that can often pass barely noticed by enthusiasts with familiar carriers such as Jet2 and Thomas Cook doing the honours. And the return arrivals tend to land during the night.