Crystal Palace manager Patrick Vieira has suffered a 'big loss' due to being unable to call upon Conor Gallagher for today's FA Cup semi-final with Chelsea, according to The Sun journalist Tom Barclay.

Vieira moved into the Selhurst Park dugout last summer and he is looking to make history by becoming the first boss to lead the Eagles to glory in the competition.

Why can't Gallagher play in the FA Cup semi-final?

Gallagher joined Palace on a season-long loan from Chelsea last summer, with him claiming Vieira played a crucial role in convincing him to head to south London.

But the central midfielder is ineligible for this afternoon's tie at Wembley as FA rules state permission will not be granted for loan players to feature against their parent club.

According to MailOnline, Palace did still head to Stamford Bridge to seek Chelsea's blessing to play Gallagher, with Vieira holding a 'slim' hope that the west Londoners would approve.

But the report suggests the Blues turned down the Eagles' request and Vieira admitted the decision was 'frustrating' for the £22.5million-rated star's development.

Meanwhile, Goal have revealed that Palace have enquired about sealing Gallagher's permanent switch to Selhurst Park.

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What has Tom Barclay said about Gallagher?

Barclay believes Gallagher's ineligibility is a major blow to Palace's hopes of reaching the FA Cup final.

The journalist feels the 22-year-old's work rate has been a big reason for the Eagles' success this season.

Barclay told GIVEMESPORT: "I do think Gallagher's energy in the press and closing down is so key to the way they play.

"He just gets at players, puts pressure on the opposition players and opens up space for his team-mates.

"I think that's going to be a big loss. It would be in any game, but particularly against Chelsea."

Why is Gallagher's unavailability a blow for Palace?

It is fair to say that Gallagher's loan switch to Selhurst Park has paid off for all parties, with him providing eight goals and five assists in 32 appearances.

His threat in the final third has got Vieira dreaming of finishing in the Premier League's top 10 during his first campaign at the helm.

Gallagher's impressive form for Palace has also resulted in him winning his first three international caps, boosting his chances of bagging a place in England's squad for the World Cup later this year, so it is clear that being unable to call upon the midfielder is a frustration for the Eagles.