Barcelona coach Hansi Flick said he feels guilty for letting Fermín López join up with Spain U21s earlier this month after the midfielder was ruled out for another three weeks.
López, 21, picked up a knock while on international duty with Spain at the start of September. He returned to light training on Tuesday, but has now been ruled out until October with a thigh problem.
The spate of injuries come on the back of a summer which saw López represent his country in the European Championships and the Olympic Games, winning both tournaments and claiming the Golden Boot at the latter.
"With Fermín, it's a real pity," Flick said in a news conference ahead of Thursday's Champions League game against Monaco. "The guy had two tournaments. Normally I feel a bit guilty because I haven't spoke with the coach with the U21s, because normally you have to say 'Hey, leave him here [with Barça].'
"But I am new [in the country]. It's not an excuse, but I feel very guilty about that."
The loss of López -- who scored 11 times in his debut campaign with the first team last season -- adds to Barça's problems in midfield with Dani Olmo also ruled out for a month after injuring his hamstring in Sunday's 4-1 win at Girona.
Gavi, Frenkie de Jong and Marc Bernal also remain unavailable in the middle of the park due to injuries, yet despite all that, Barcelona top LaLiga with a perfect 15 points from five games.
"Fermín's a player we need in this situation," Flick added. "He can score goals and he is very professional. [On Tuesday], when I saw him with the doctor, it was not nice. We have to take care of the players, because sometimes it's too much.
"Gavi and Frenkie are back in training but they are not 100%. We have to take care with them.
"It will be difficult to [directly replace] Olmo because he has quality with the ball. In front of goal, he can score, give the last pass and he's calm with the ball."
Flick has called up 16-year-old cousins Toni and Guille Fernández to help deal with the injury crisis, which defender Jules Koundé believes is down to the growing number of games on the football calendar.
The France international echoed Manchester City midfielder Rodri's suggestion that players may have to go on strike because of concerns over their increasing workload.
"There are different factors [for the injuries], but the load of games and fatigue is one of the reasons and does not help," Kounde said in a news conference on Wednesday.
"I agree with everything Rodri said. The calendar is getting longer every season, there are more games every year and less rest. But we have been saying this for years and no one listens to us, the players, who are the main actors.
"Perhaps we need to go on strike to make our voices heard. We are already taking too many risks and there are more and more injuries, regardless of how well we prepare for games.
"This year, there will be players in the Club World Cup at the end of the season, too, who could reach 70 or 80 matches. It's crazy.
"We need to organise ourselves. There are several unions, but I think we need one message that represents all footballers. It's something we will have to discuss among the players."
Further forward, Ansu Fati returns to the Barça squad to face Monaco, who beat the Blaugrana 3-0 just over one month ago in the Joan Gamper Trophy at the Olympic Stadium.
Flick says Fati is someone who will add goals to the team, played down the significance of that defeat to Monaco and said, after five wins in five in LaLiga, his team are ready are ready for this week's European bow.
"We showed in recent matches how serious we play and how focused we start games," he added. "Girona was a match at Champions League level. With the ball, Girona are very good, but with our pressure, they suffered a lot and we scored four goals.
"Thursday's match is against a very good team, they beat us in the Gamper, but now is a new situation. This is the best competition in the world. Everyone is ready, wants to play and hopefully it goes in the right direction.
"A club like Barcelona, big clubs, always want to win titles. The Champions League is the best competition and we enjoy all this and play on the highest level. We have to be ready to play against the best teams. If you are not focused, not 100%, you have no chance in this competition."