BARCELONA -- Ernesto Valverde says it will be difficult to replace the injured Sergio Busquets against Athletic Bilbao this weekend, labeling the midfielder "the best in the world" at what he does.

Busquets, 29, broke a bone in his foot in the Champions League win against Chelsea on Wednesday and will be sidelined for the next three weeks.

He is one of five players missing for Athletic's visit to Camp Nou on Sunday. Thomas Vermaelen, Nelson Semedo and Denis Suarez are also out through injury and Luis Suarez is suspended.

As a result, Valverde has called up Barca B midfielder Carles Alena, who he said has a "very big future" at the club.

"It's always bad to lose players of [Busquets'] calibre," Valverde said in a news conference on Saturday.

"Busquets is fundamental for the team. He is the best player in the world in his position, so it's not easy to replace him, but there are alternatives -- [Ivan] Rakitic has played that role very well.

"Rakitic is having an extraordinary season. Maybe his role isn't as attacking as it was at Sevilla, but players do move around. They don't always stay in the same position. Rakitic is a very important player for us."

If Rakitic drops into Busquets' position, there could be a first La Liga start of the campaign for 20-year-old academy graduate Alena in a more advanced role.

Alena's three appearances to date this season have all come in the Copa del Rey, but with several players missing with injuries and Andres Iniesta only just back from a hamstring strain, the chances of him starting against Athletic have increased.

"Iniesta is a unique player. You can't compare him with anyone in the game," Valverde said when asked if Alena, who scored one and made another in the B team's 2-2 draw with Numancia last weekend, could be the eventual heir to the current Barca captain.

"But I can say that Alena is a player with a very big future at this club -- there is no doubt about that."

Sergio Busquets will miss at least three weeks with a broken bone in his foot. Etsuo Hara/Getty Images

Elsewhere, there could be a start for Paco Alcacer up front in place of the suspended Suarez, with Lionel Messi and Ousmane Dembele expected to keep their places in the side.

Barca remain unbeaten in the league this season and are eight points clear of second-placed Atletico Madrid, but Valverde urged his players to maintain their focus as the "final countdown" begins with a special game against his former club.

"Athletic are always a dangerous opponent," he warned anyone thinking the Basque side, who are 12th in La Liga, would roll over on Sunday.

"It's a special game for me, but at the same time, it's a dangerous one because Athletic always have a strong sense of spirit and the three points are really important for both teams.

"The final countdown has begun. There are only 10 games left. We want to go into the international break with a good result because we've got a busy April coming up and three trophies in play. Our season could boil down to that one month."

One of the three trophies in Barca's pursuit of a third Treble is the Champions League, with the Catalans paired with Roma in the quarterfinal draw on Friday.

Barcelona avoided the likes of Real Madrid, Manchester City and Bayern Munich in the draw, but Valverde is wary of the threat posed by the Serie A side.

"Roma are a tough opponent for a lot of reasons," he said. "Because of what they've done, what they've won, they're in the last eight of the Champions League, they have a very good coach, [sporting director] Monchi knows us very well, they beat Chelsea, knocked out Atletico Madrid...

"I have a huge amount of respect for them. All this about them being the best opponent for us to get, I don't believe that. They will be a very tough rival."