Welbeck's teammates' reactions spoke volumes as he was attended to by physios on the pitch at the Emirates.
However, the former Manchester United striker - dubbed by the Portuguese sportspages the "Welbeck Express" such was his all-round powerful display in Lisbon 14 days previously - suffered what looked to be a serious injury in the 25th minute.
He was taken off on a stretcher with his right leg heavily strapped before being rushed to hospital to assess the damage.
"The worst news tonight is these injuries", the Spaniard told a media conference.
"His injury is also [bad] for the mentality of the players because they looked and saw it is a big injury, but when you are playing football and competitive matches it can happen".
Speaking after the final whistle, the Arsenal boss said: "I think Danny was working well".
The injury casts Welbeck's club future into further doubt, with the 27-year-old out of contract at the end of the season.
"It's very challenging mentally, but sometimes it comes in the game and there's been a lot of injuries, and unfortunately for Danny he's had a few so we're going to wish him the best", he stated.
Whether Welbeck will play again in 2018/19 remains to be seen. But when the match finishes, the first thing they did was ask for him.