Arsenal are 2-0 up in their quarter-final tie against Napoli after last week's opening leg at the Emirates Stadium.
But Emery will need his strongest side for the trip to the Stadio San Paolo if the Gunners are to book their spot in the last four.
Torreira, 23, was replaced by Mesut Ozil at half-time against the Hornets as they picked up three points to return to the Premier League's top four:
Emery explained that he substituted the midfielder partially for fitness reasons, but also because the Vicarage Road crowd were getting on his back after he was victim of an elbow in the face that saw Troy Deeney sent off early in the match, per Chris Harris of Arsenal.com:
"First I decided to do the change of Lucas Torreira because after the red card, the atmosphere is against him and also the doctor said to me in the first half that he has a little pain for Thursday, and I decided 'okay, it's the moment' [to take him off].
"At the end the doctor said to me that they are okay. Only in the first half with his small injury, Lucas Torreira, but I think it's small and he can be ready for Thursday physically. It was a very physical match today, but I think we can respond well and above all the result gives us the opportunity to continue in our way, being consistent in the table.
"It's the same problem as Thursday, it's a small pain and I think [Torreira] is going to be okay for Thursday."
Arsenal's win at Watford was scrappy but welcome. Pierre-Emerick Aubameyang got the only goal of the match in the 10th minute as he took advantage of a rare error from goalkeeper Ben Foster:
Arsenal have spent the last two seasons in the Europa League, but a top-four finish would see them return to UEFA Champions League action next term.
They must keep picking up points on the road, though, as three of their last five league matches of the 2018-19 season are away from the Emirates.
The Europa League also offers Arsenal a path back to the Champions League as they will be granted a spot in Europe's elite tournament if they win it.
Napoli pose a big threat to their progress, but Emery's side are in an excellent position following the first leg.
Arsenal will face Villarreal or Valencia in the semi-finals, assuming they do progress, and then Premier League rivals Chelsea could possibly await in the final.