The hosts are one of four clubs on 10 points heading into the fifth round, and they aim to dispatch their fourth-bottom opponents keen to respond after a below-par start.
Match preview
Four games into his second year in charge, and Kily Gonzalez is probably loving life in Santa Fe after a difficult first season that saw Union end 20th in the Primera Division.
While the new campaign is still in its infancy, an unbeaten start leaves El Tate fifth in the standings, joint-top with Racing Club, Talleres, Huracan and Gimnasia — Gimnasia have played their fifth game at the time of writing.
Gonzalez's team rank fifth in the table due to their inferior goal difference but could still supplant those clubs if they beat San Lorenzo this weekend and their early-season challengers drop points.
That outcome may be a long shot for Union, even if supporters in Santa Fe have reason to be optimistic as their side typically get over the line with one goal.
One goal has been enough for El Tate in 50% of their four games, while a 1-0 triumph against Barracas Central was in the offing until Jeronimo Domina's second-half stoppage time made it 2-0.
That defensive astuteness poses problems for San Lorenzo, whose absence of firepower does not bode well for their prospects in gameweek five.
Los Santos have netted once in three games — their match at Godoy Cruz was suspended after a fight broke out — losing twice against Deportivo Riestra and Sarmiento (both 1-0 losses), with their only point after three tries a 1-1 home draw with Lanus in May.
Nonetheless, Leandro Romagnoli's men enter Saturday's fixture on the back of a 2-0 Copa Argentina win over Chacarita Juniors, and they hope to carry that confidence into facing their unbeaten hosts.
With Copa Libertadores progress assured in their final group match, Los Santos hope to get back on track domestically and rise in the standings after a disappointing start that has left them with one point from a possible nine.
Last season's third-placed side could slide to third bottom if Tigre better their result as both teams have the same point although the 26th-placed side have completed four games to San Lorenzo's three.
Union Argentine Primera Division form:
W
D
W
W
San Lorenzo Argentine Primera Division form:
L
D
L
San Lorenzo form (all competitions):
Team News
Bruno Pitton (two yellows) and Adrian Balboa (straight red) are suspended after dismissals in Union's 1-0 win over Independiente Rivadavia at the start of the month. Balboa was also the game's match-winner after his 63rd-minute penalty decided the clash.
Mauro Luna Diale and Francisco Gerometta (both knee) are expected to miss out, along with Patricio Tanda, who has a muscle problem.
Gaston Gomez (muscle) is absent for several weeks for San Lorenzo, while Manuel Insaurralde and Gaston Hernandez have severe knee injuries.
Alexis Cuello netted two goals in three starts in the Copa Libertadores, but it remains to be seen if Romagnoli hands the forward his second start this term.
Cristian Ferreira, scorer of the Saints' only goal after three matches, should play from the off supporting central striker Cristian Tarragona.
Union possible starting lineup:
Cardozo; Paz, Pardo, Torren; Vera, Pitton, Rivero, Mosqueira, Corvalan; Domina, Orsini
San Lorenzo possible starting lineup:
Clemente; Giay, Lujan, Campi, Braida; Perruzzi, Tapia; Cerutti, Ferreira, Barrios; Tarragona