Sevilla have picked up just one point from their last four fixtures in the Primera Division and were somewhat fortunate to get even that after needing a last-minute equaliser to salvage a 2-2 draw against second-bottom Espanyol.
That poor run has seen Michel's men slump to 13th place in the standings, six points above the relegation zone, ahead of their trip to San Sebastian.
However, Montanier believes those results do not tell the whole story, pointing out the fact Sevilla were hamstrung by at least one red card in each of those four matches.
In the last three games where Sevilla have finished with their full quota of players, they have beaten Real Betis 5-1, Espanyol 3-0 and Real Mallorca 5-0, the latter two games coming in the Copa del Rey.
Montanier told reporters: "The last few games I've seen of theirs are not a guide because they've finished with nine or 10 players. I think the level they showed against Betis and Real Madrid (1-0 win) was very high.
"At those times you saw that they have the potential of a Champions League team."
Sociedad are ninth in the standings and unbeaten in seven matches in all competitions, but Montanier knows they will need to improve on their last performance, a goalless draw at Granada, if they are to beat Sevilla.
"We need to play with more speed, better as a team with more mobility up front, because if not we aren't going to beat anybody, as was showed in Granada," he added.
Sevilla goalkeeper Diego Lopez said of Thursday's match, which is their last before the winter break in Spain: "It's a very important game for the team and the club, we're going there with the mentality of turning things around.
"It's a difficult game, at a tough ground, but our play and capabilities means we can win there. We're working very well, things aren't turning out how we wanted, but our work and confidence hasn't been dented."