The Tottenham striker was unavailable for last night’s first leg, but is set to be involved in the return leg next Wednesday (8pm).
He was sorely missed at Tottenham’s new stadium as Spurs struggled going forward and lost 1-0.
In his place, Mauricio Pochettino was forced to start Fernando Llorente and Lucas Moura up front.
But Ten Hag is not getting too carried away, despite Donny van de Beek’s goal giving them a 1-0 advantage going into the second leg, because he is well aware of the talents of the South Korean.
“Of course, he’s a very good player,” he said when asked if Son will change the way they approach the second leg.
“When he’s not available for Spurs, that’s a very big disadvantage for them.
“But of course, we also have very good players to defend against him, and we will do.”
The game was somewhat overshadowed by an injury suffered by Jan Vertonghen in the first half.
The former Ajax star went up for a header alongside Toby Alderweireld, and the two team-mates clashed heads while attempting to meet Kieran Trippier’s free-kick.
Vertonghen was brought off, before returning to the pitch minutes later under controversial circumstances.
Despite it seeming that he was unfit to return, it was deemed that he could return to the field, and he was subsequently taken off again moments later after looking visibly disorientated.
And when asked about how to deal with head injuries, Ten Hag declared that managers need to be careful about taking chances.
“They are the rules that we play with,” he told BT Sport.
“What I want to say is that with a head injury, it can be dangerous so get him off.
“Don’t take risks with head injuries, and that’s what they did [not take risks].”