He was set to face Leicester in the Gallagher Premiership on Sunday before he got the call to replace Willi Heinz, who suffered a hamstring injury in the semi-final win over New Zealand.
It means Spencer will be named on the bench for Saturday's World Cup final and given he is the only other scrum-half in Eddie Jones' squad, the chances are he will be needed against South Africa.
If the 27-year-old is required in Yokohama, it will be only his fourth international appearance, but he will be familiar with the opposition. Spencer made his debut in June 2018 in Johannesburg in a thrilling contest where England lost a 21-point lead to lose 42-39 to South Africa. A five-minute cameo was not long for the Saracens scrum-half to make an impact and it was a similar story in his second appearance a week later. Jones introduced Spencer in the 72nd minute but South Africa won again, this time 23-12, to heap more pressure on the head coach. The only other time the former Cambridge player made it onto the pitch for his country came late on in the 38-38 draw with Scotland in the Six Nations earlier this year, so he is yet to win in England colours.
Spencer does play far more frequently for Saracens, though, and has appeared more than 150 times for the Premiership holders since his debut in 2011. Last season he played a key part in their European and domestic double, impressing in the Heineken Champions Cup semi-final win over Munster and scoring tries to help them beat Gloucester and Exeter in the play-offs to win another league title. He will now hope to add the Webb Ellis Trophy to his honours list and conclude a wonderful 12 months which has seen him displace Richard Wigglesworth as the starting scrum-half at Allianz Park.
Although Spencer will not be wearing Saracens' black and red, he could be forgiven for thinking he is on tour with his club in Japan due to the amount of Sarries in the England squad. He is one of nine members of Jones' 31-man squad who play their rugby at Allianz Park and will be welcomed into the group by captain Owen Farrell in particular. The duo were central to Saracens' strong and successful end to the season and Spencer will feel at home with Jamie George, Jack Singleton, Mako Vunipola, Maro Itoje, George Kruis and Billy Vunipola also in Japan plus Elliot Daly, who joined Sarries from Wasps this summer.
Stockport-born Spencer is rapid and his excellent running game is known by many, but his game control has noticeably improved in recent seasons. This has seen him earn international recognition and he is a scrum-half who is not afraid to show off his kicking accuracy. In the 2015 Anglo-Welsh Cup final, Spencer kicked a last-minute penalty to guide Saracens to a 23-20 victory over Exeter. With an eye for the try-line, the former England Saxons player will be confident of making a mark on the biggest stage if required against the Springboks.