Atlanta United centerback Miles Robinson will miss Tuesday’s game against Motagua in the Champions League because of a left quad injury, manager Frank de Boer said on Monday.

Additionally, fullback Edgar  Castillo (rib), fullback George Bello (concussion), winger Jake Mulraney (visa), midfielder Matheus Rossetto (visa) and winger Manuel Castro (travel) aren’t available for selection.

“Makes it a little bit thin for me,” de Boer said.

Robinson’s injury happened during last week’s friendly against Leones Negros in Mexico. De Boer said Robinson sustained the injury while stretching after feeling some discomfort.

Bello’s injury happened two weeks ago in a friendly against Birmingham Legion.

Castillo’s injury happened three weeks ago in a friendly against Swedish side Elfsborg. He suffered broken ribs and a collapsed lung last year while with New England. De Boer said Castillo feels pain when he turns.

It doesn’t seem likely that the absences of Robinson and Bello will mean that rookie George Campbell will be in line to start at centerback.

“He’s a very promising player,” de Boer said of Campbell, who has played a lot in the past two friendlies.

De Boer said he has spoken to Campbell about the need for increased intensity.

“In the future, he will be ready, to start directly and to put pressure on him ... that’s the question for the coach,” de Boer said. “During the season, you think this is a great opportunity to get used to the pressure and environment. I’m doubtful he can do that (on Tuesday) but in the future he will be an important player for Atlanta United.”

If neither Robinson nor Campbell starts, it seems probable that Atlanta United will go with a four-man backline using two centerbacks and two fullbacks. The centerbacks would likely be Fernando Meza and Franco Escobar, with Brooks Lennon as the right fullback and Anton Walkes as the left fullback.

De Boer said he expects that Robinson, Bello and Castillo will return soon, so he’s not panicking over the need to make another signing.