Jefferies had previously indicated the Portuguese centre-back would not feature because he had refused to sign a new deal, with his existing contract due to expire this summer.
But robbed of 14 first-team squad members, the Hearts boss felt he had no choice but to send an SOS to the 24-year-old, who had not previously played in 2010.
That was done after consultation with sporting director Aleksandr Metlitski, amid reports permission to play Goncalves had been sought from majority shareholder Vladimir Romanov.
Goncalves - who shook off a knee injury to start yesterday - was the visitors' only recognised centre-half on the field, with Wallace forced to move from left-back to fill in alongside him.
"It was a bit of a surprise the fact that he was going to play," said Scotland star Wallace, who captained the side at Tannadice.
"He was carrying a knock. I think he trained maybe two days.
"It shows his fitness and his desire and his commitment to the club.
"He's come back in and he's a rock at the back. It was a big plus for us."
Asked if he would like Goncalves to be allowed to remain in the team, Wallace said: "Of course - he's a key player for us. He's showed that all season.
"Off the park - whatever the issues are there - it's for that to take care of itself.
"We want to see him remain here and remain playing for the rest of the season."
Wallace, who was sporting a bandage to cover stitches in a head wound, said of his own centre-back adventure: "The manager said to me, 'How do you feel about playing there?'
"I said, 'I'm happy to play anywhere'. I'm happy to play there again next week."
Wallace insisted he and his team-mates would not use Hearts' injury crisis as a "get-out clause" for their three-match winning run coming to an end.
"We're not going to sit and make excuses because of all these injuries," said Wallace, who pointed out Paul Mulrooney shone yesterday despite only starting because captain Michael Stewart failed to complete the warm-up.
"He was fantastic. He proved there's a great youth policy at this club.
"Although there are a lot of players missing, it does show the character and the commitment of everyone else."
Sunday's defeat opened up an 11-point gulf between Hearts and third-placed United, severely denting their European ambitions.
"We've just got to stay positive," Wallace said. "We lost the game here but it's on the back of three wins."
United's winner was scored 13 minutes from time by Morgaro Gomis, his third goal in two games.
It saw the Tangerines regain a one-point lead over Hibernian, with Motherwell a further three back.
Gomis said: "Motherwell and Hibs are good teams - they're still going to carry on, try to win every single game.
"That's what we're going to have to do as well, so it's going to be good."
United's cause should be helped by the return of last season's top scorer Francisco Sandaza, who came off the bench yesterday for his first appearance since summer hernia surgery.
Despite his own rich vein of goalscoring form, Gomis said: "I don't think I can give anyone any tips! But hopefully he's going to score a few goals."
Source: Team Talk
Source: Team Talk