Yes, Americans were very concerned with the arrival of the Su-27. It's a big, fast, long-range, and maneuverable jet that can carry 10-12 air-to-air missiles, configuration dependent. One of the biggest worries was that the Flanker's radar had a multi-target tracking and engagement capability like the F-15 and F-16 had. Combine that with the new long-range AA-10 ALAMO missile (although NATO overestimated its actual range) or developmental R-77/AA-12 active radar missile, the Flanker was looking like an equal to the F-15 and F-16 in terms of air-to-air capabilities.