From Longman Dictionary of Contemporary EnglishI can only think/suppose/assume (that)I can only think/suppose/assume (that)spoken used when you are giving a reason for something, to say that you do not know something for certain but think that this is the only possible reason I can only assume that it was a mistake. → only
Examples from the Corpus
I can only think/suppose/assume (that)• As for an Iguana Air, I can only assume it's a tropical airline.• Now if I can only think of their name. 