Analyst Jason Ader cited "signs of tightening demand across the IT infrastructure universe" at the networking giant.
Shares were down 1.64% to $56.17.
Lower demand "could pressure growth in Cisco's fiscal 2020, especially when compared against unusually strong demand in fiscal 2019," the analyst said.
In a little more than a decade, William Blair has rated Cisco outperform twice and market perform twice. Historically, shares gained an average 80% when it was rated outperform and 33% in the periods it was rated market perform.
Upside to consensus 2020 expectations, "as well as multiple expansion, will be more challenging from here," Ader wrote in a note to clients.
The stock has 20 buys, 11 holds and two sell ratings, according to Bloomberg. Its average price target is $57.
Cisco shares have risen more than 40% from their December low.