MANCHESTER: An injury crisis, coupled with an intense schedule, is eroding Manchester United's status as favorite to win the Europa League.

United, England's most decorated club, is the standout name on the list of semifinalists that also contains Celta Vigo, Lyon and Ajax.

Yet manager Jose Mourinho's ambitious attempts to mount an assault on every competition this season — domestic and European — is starting to backfire and United is limping into its first leg against Celta in Spain on Thursday.

On Sunday, defenders Eric Bailly and Luke Shaw joined an injury list that already included Zlatan Ibrahimovic, Paul Pogba, Marcos Rojo, Chris Smalling and Phil Jones. Four of those are center backs, leaving Mourinho without a fit, recognized senior central defender.

Fatigue is also setting in with United having played 57 games already this season, nine in April alone.

"We are tired and exhausted," said Mourinho, who described his team's schedule as "not human." ''We have a squad of 22 that is reduced to 13 or 14 players.

"There are high levels of fatigue," he added, "big match after big match."

While United's patched-up team was toiling in a 1-1 home draw against Swansea in the Premier League this weekend, Celta was resting its top players for a 3-0 home loss to Athletic Bilbao in the Spanish league. There's no doubt which team will be fresher on Thursday.

Ajax plays Lyon in what appears to be an evenly matched semifinal, with the first leg in Amsterdam on Wednesday. For Lyon, like United, winning the Europa League is the best route to qualify for next season's Champions League, heightening the importance of its progress in European soccer's second-tier competition.

A closer look at the first legs:

AJAX vs. LYON

Lyon has been given an unexpected boost with top scorer Alexandre Lacazette recovering from a right hamstring injury earlier than expected.

The striker was hurt late in the second leg of Lyon's quarterfinal match against Besiktas, which ended with Lyon advancing via a penalty shootout. It remains to be seen whether Lacazette, who has 31 goals this season, will start in Amsterdam or be on the bench.

In a further boost to Lyon, key midfielder Corentin Tolisso was also named in the squad after going off with a knock in Friday's 2-1 win at Angers in the French league. The availability of Lacazette and Tolisso gives Ajax coach Peter Bosz more to think about as he prepares his defense to face one of the most attack-minded sides in Europe.

Ajax, a four-time European champion, will be missing two key defenders, too, in Joel Veltman and Nick Viergever because of suspension. Another defender, Daley Sinkgraven, and striker Bertrand Traore are in doubt with injuries.

With Feyenoord closing in on its first Dutch league title since 1999, the Europa League looks to be Ajax's last chance of silverware this season, although the team looks likely to secure entry to the Champions League qualifiers courtesy of a second-place finish.

It is Ajax's first semifinal in European competition since 1997.

CELTA VIGO vs. MANCHESTER UNITED

Playing in its first European semifinal, Celta is barely known outside Spain but has established itself at home as one of the league's most attractive teams.

A coach steeped in the attack-first tactics, Eduardo Berizzo has maintained Celta's scoring flair despite losing forward Nolito to Manchester City last year and midfielder Fabian Orellana to Valencia in January.

Celta's focus on scoring means the team has exposed its defense to some beatings this season, while also producing major upsets.

Before Orellana's departure, Celta beat Barcelona 4-3 at home, then stunned Real Madrid in the Copa del Rey, winning 2-1 away before earning a 2-2 draw to advance to the semifinals. Without Orellana, it has eliminated Shakhtar Donetsk, Krasnodar, and Genk in the Europa League's knockout rounds.

Iago Aspas, who failed to impress in a spell at Liverpool, is thriving once again at his boyhood club. The striker leads Celta with five goals in the competition, followed by forward John Guidetti's four.

But the key to Celta's fortunes rests on its playmaking midfield unit led by Daniel Wass and Pablo Hernandez.