With the second-half only a few minutes old, and England now trailing by two goals to one, Shaw appeared to catch his head on Carvajal's leg.
Danny Rose came on for Shaw at left-back as the tempo of the match slowed immediately following the Manchester United player being carried off the field, with neither side fashioning any real chances in the following 15 minutes.
However, Shaw's condition is not as bad as first feared, with the left-back posting on Twitter an hour after the match ended 2-1.More news: PLAID SHIRT GUY? Look on teen's face at Trump rally raises eyebrows
"He is sat up in the dressing room talking and seems fairly comfortable", Southgate said.
"We'll have to assess him a little bit more overnight and tomorrow, but that's definitely a good sign".
Southgate also said it was "difficult to tell" if Shaw had lost consciousness on the pitch after the collision, adding: "We'll go through some tests". "It was a nasty looking fall and collision".
Shaw started for the Three Lions at Wembley and set up club teammate Marcus Rashford for the first goal of the match.More news: Egypt court sends 75 to death, 47 to life over 2013 protest
"Great contribution for the goal", the England manager said. For him the problem was psychological and he needed games like that at Wembley.
"We have got quite a few players short of minutes and a bit of sharpness, so to come into a game like tonight after so long out of the global scene is as tough a test as you can have".
There were nine minutes of stoppage time to take into account the time Shaw was being treated, but try as they might, there would be no late comeback for England, as a very late strike from Welbeck was ruled out for a supposed foul on De Gea.More news: Donald Trump Mocks Obama Shellacking , Demands DOJ Investigate NYT Op-Ed Writer