United will be looking to build on the victory at Burnley just before the global break when they take on the in-form Hornets, but could have to do it without Shaw and Rashford.
Shaw, who started his first match for England since March 2017, suffered a blow to his head as he landed awkwardly after a challenge against Spain's Dani Carvajal, which saw him substituted after 57 minutes.
Play had to be stopped for around 10 minutes and he was treated by 10 medical staff before being carried from the field on a stretcher, in a neck brace and being given oxygen.
Players, colleague and fans were anxious about Shaw, but hours later, the youngster thanked everyone, and stated that he is a fighter and will be back soon.
"I'm a fighter so I will be back soon", his tweet further read.
"He is sat up in the dressing room talking and seems fairly comfortable", Southgate, as quoted by The Indian Express said.
Manchester United and England will hope his latest injury is not a major one and he recovers quickly.
"Great contribution for the goal.to come into a game after so long out of the global scene is as tough a test as you can have".
Gareth Southgate's side started their maiden UEFA Nations League campaign with a frustrating 2-1 defeat on a night when concerns over their 23-year-old left-back's well-being overshadowed proceedings.
"I thought Dele's performance was good against Spain".