The Red Devils slipped up in their hopes of securing a top-four finish in the Premier League at the weekend as they were thrashed 4-0 by Everton.
With Manchester City and Chelsea both visiting Old Trafford this week, United could well see the door shut on their hopes of playing in Europe’s premier competition next term.
United have played in the Champions League in both of the last two seasons since winning the Europa League in 2016-17.
The club still managed to sign Paul Pogba in a then-world record £89million move despite heading into that season without Champions League football to look forward to, also bringing in Zlatan Ibrahimovic, Henrikh Mkhitaryan and Eric Bailly in deals that were high profile at least at the time.
But former Premier League striker Kitson believes failing to secure a spot in the tournament for 2019-20 would have a harsher impact on United in the transfer market this time around, with Ole Gunnar Solskjaer tasked with overseeing a huge squad overhaul.
And in particular, he thinks Barcelona and Manchester City-linked wonderkid De Ligt, of Ajax, would snub them if they failed to secure a top-four spot.
Kitson told Malaysian broadcaster Stadium Astro: “He’s got to get to the Champions League because there’s going to be certain players… They need a centre half, De Ligt at Ajax for example, €100m (£86m).
“They could have gone out and got him. If they don’t get into the Champions League, that’s an 18-year-old kid they attract anymore.
“The Champions League is massive for them.”
United’s Goodison Park reverse leaves them three points off third-placed Tottenham after Spurs, Chelsea and Arsenal all dropped points over the same weekend.
But Kitson was uncomfortable with the manner of United’s performance on Merseyside given their tussle for a top-four spot.
On the travelling United fans continuing to clap and cheer as Solskjaer apologised at full-time, he said: “They were good, they sung until the last moment, they were determined to show their support.
“They’ve always been decent fans, United, they’ve always given everyone a fair crack of the whip.
“It’s fine pointing out what went wrong but I want to know why, I want to know why you can’t get a group of footballers at United to give the same effort as Everton.
“I would’ve said it’s an end-of-season performance but they’re fighting for the Champions League and that’s the most disturbing thing about it.
“I think there’s a culture at the moment where one person is going to wait for the next person who is going to wait for the next person to deliver performances and results.
“And at this moment in time they are not the team to be able to do that.”
United host neighbours Manchester City tomorrow night (8pm) with their rivals looking to use their game in hand to steal a march in the Premier League title race over Liverpool. City will move one point clear at the summit with a win at Old Trafford.