Rashford had a disappointing end to United's Premier League season, and Southgate felt he may have been trying too hard.
Rashford played in Dele Alli and his low cross found fellow substitute Welbeck at the far post.
England start their World Cup Group G campaign in Russian Federation against Tunisia on June 18, while Costa Rica open against Serbia, a day earlier, in Group E.
If Pope gets on, and with Trent Alexander-Arnold set also certain to feature tonight, England will head into the World Cup with no uncapped players in their squad. England outpossessed Nigeria 59-41, with each team having 12 shots, four on target. "It's good practice for what we need in the tournament".
"We'll definitely take the things that happened in that tournament into the next one because that's going to put us in good stead".
Manchester United mean business, sign Brazilian midfielder Fred
At around 5 ft 7 inches, Fred is a dynamic box to box midfielder who should improve United's tempo going forward. That was when coach Jose Mourinho came into action.
England comes into this match unbeaten in their last nine games, including a narrow 2-1 win from Wembley over a very game Nigerian side on Saturday.
Navas spread himself well to deny Jamie Vardy from close range as England threatened a quickfire second, with Jack Butland, widely-expected to be back-up to Jordan Pickford, springing into action to deny Johan Venegas.
"Anything's possible so we have to go there with the highest expectations".
England play Costa Rica in an global friendly before the World Cup 2018 in Russian Federation.
Costa Rica face tournament favourites Brazil as well as Serbia and Switzerland in Group E, and will need to show far more ambition once they get to Russian Federation if they are to replicate their heroics of four years ago.
Manchester United fans and England fans were absolutely blown away by Marcus Rashford after he netted a stunner against Costa Rica.