Manchester United manager Jose Mourinho said initial news over injuries to Zlatan Ibrahimovic and Marcos Rojo was "fairly negative".
But the 3-2 aggregate success came at a price, with Ibrahimovic and Rojo both seemingly set for extended spells on the sideline after being taken off.
"I don't think they are easy injuries but I prefer to wait until all the tests are done and then speak. I can't do that because I am a manager not a doctor," Mourinho told BT Sport.
"But the news is fairly negative I think."
Henrikh Mkhitaryan again opened the scoring for United, as he did in Belgium, but his goal was cancelled out by Sofiane Hanni.
It took until the 107th minute for the hosts to secure their win through Marcus Rashford, who had been goalless in the Premier League since September before netting in back-to-back games earlier this month.
"You speak about his qualities and they are fantastic. But mentally he is excellent," Mourinho said.
"He hadn't scored since September, some players don't understand what I want but Marcus I trust.
"It doesn't matter if he doesn't score - he is fantastic in his effort and attitude."