Atlanta United may be without one of its best players when it plays Monterrey in the quarterfinals of the Champions League on Wednesday.

Wingback Julian Gressel, who tied with Miguel Almiron for the team lead in assists (14) last season and is tied with Josef Martinez for the lead in goals (2) this season, is doubtful for the first leg at Estadio BBVA Bancomer.

Manager Frank de Boer said Gressel suffered an ankle injury during training on Monday.

“Not clear if he can play. Hopefully he can,” de Boer said.

Gressel was held out of Sunday’s loss at D.C. United. Mikey Ambrose started in his place. Franco Escobar, who would likely start in place of Gressel, is still recovering from a shoulder injury. Gordon Wild could also be considered by de Boer to start.