Romero was forced off after colliding with goalkeeper Antonin Kinsky, leaving the pitch in tears.
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Afterwards, De Zerbi conceded: “Maybe yes (it is a knee injury), but I don’t want to say anything until we know more.
“We don't know yet (the extent of the injury).
“We have to see in the next few days. I hope it's not an important problem because he's a crucial player for us.
“A good guy, a top player, big personality, and we need him to finish the season and to achieve our goal.”