Complex sentence: Aris had to wait for another year to compete in Europe again, but his strength had been established: in 1987 ARIS were not paired against an established team and thus advanced to the final round of the 8 best european champions.
Split into simpler sentences: Aris had to wait for another year to compete in Europe again, but their strength had been established. In 1987 Aris was not paired against an established team and thus advanced to the final round of the 8 best European champions.