Having claimed a crucial win over Benetton last time out, Ulster face Scarlets on Saturday in the midst of a fierce battle between several teams for the eight URC play-off spots.
With three regular season games remaining, only six points separate Stormers in fifth and the Lions in 11th.
And Soper believes that this weekend’s contest at 14th-place Scarlets is “definitely” a must-win game for Ulster ahead of tough interpros against Leinster and Munster.
“We’re at three games left and it’s so tight, it’s incredibly tight for the top eight,” said Soper.
He added: “The Scarlets have had a tough enough season and on the face of it everyone expects that we’d go over there and do a job.
“But I think it’s going to be tough.”