Losing 3-1 to Leicester in the FA Cup quarter-finals on Sunday leaves Ole Gunnar Solskjaer with realistically only the Europa League to produce his first trophy as United manager.
United faces Granada in the quarterfinals of the Europa League.
“We didn’t have the spark tonight and it’s understandable,” Solskjaer said.
“They’ve been unbelievable, the last three or four months, playing every three or four days, and it’s just caught up with us — all the games, all the travelling.
“Thursday night in Milan took a lot out of us, today we didn’t have that extra zip.
But with United second in the Premier League, albeit 14 points behind Manchester City, securing Champions League qualification could be the most significant achievement.
It will be lucrative, too, and for Leicester, which is only a point behind United in third.
But while Brendan Rodgers knows the value of steering Leicester back into the Champions League for the first time since Claudio Ranieri’s side won the Premier League in 2016, the status of a trip to a first FA Cup semi-final since 1982 is notable.
After Kelechi Iheanacho’s double eliminated United, Leicester has the easier route to the final after being drawn to play Southampton, who beat Championship side Bournemouth on Saturday.
The other semi-final will pit Man City — who fought hard to overcome a dogged Everton side 2-0 thanks to two late goals at Goodison Park on Saturday — against Chelsea, who beat Sheffield United 2-0 in Sunday’s other quarterfinal.
The FA Cup semifinals next month take place a week before the League Cup final between City and Tottenham.
That is Jose Mourinho’s only way of delivering Tottenham’s first trophy since 2008 after the north London club’s elimination from the Europa League on Thursday.
But Tottenham helped its chances of qualifying for the Champions League by beating Aston Villa 2-0 in the Premier League on Sunday to move within three points of fourth-place Chelsea.
Tottenham received some assistance from neighbour Arsenal recovering from 3-0 down to draw 3-3 with West Ham, which missed a chance to move level on points with Chelsea after conceding Alexandre Lacazette’s equalizer.