A Reuters poll published on Monday showed money market traders expect the ECB to announce 600 billion of sovereign bond purchases, but they also believe this will not be enough to bring inflation up to target.
Data released on Monday showed the euro zone's current account balance shrank in November, suggesting continued euro weakness, but analysts said the single currency could bounce if the ECB disappoints on Thursday.
Read MoreSNB shock will only cement CHF & Yen safe haven status
The euro was trading at $1.1601, up 0.3 percent on the day but not far from a trough of $1.14595 hit on Friday. It is already down more than 4 percent in January—its biggest monthly fall in 2-1/2 years.
"If they (the ECB) disappoint, there is definitely an argument that you would get an immediate covering of euro shorts and it may also lift euro area bond yields,'' said Stephen Gallo, European head of FX strategy at BMO Capital Markets in London. "But in our opinion that's a rally that you have to really look to sell into."
The Swiss National Bank removed one of the euro's few remaining pillars of support last Thursday when it stunned markets by abandoning its three-year-old cap on the Swiss franc against the euro. The move also fueled speculation that the SNB was acting in anticipation of more aggressive easing from the ECB.