Was jamming with friends today. One of the guys started playing a blues tune and said "just 2 chords on this one...D and E.

I got the nod for a lead and started playing in D major pentatonic. It sounded ok but not great.

After the song ended the guy said. "you know that song was in A."

So my question...in this case since the 2 chords were back to back, should I have figured that those were the 4 and 5 chords and the key was A, even though there was no A (or root) ever played? Thanks!