BUFFALO, New York (AP) — The NHL has blocked the Buffalo Sabres' bid to send Russian rookie center Mikhail Grigorenko to the minors on a conditioning assignment.

The Sabres announced the decision on Wednesday, a day after Grigorenko was among three rookies assigned to Rochester in the American Hockey League. Under NHL rules, teams are allowed to assign players on conditioning stints lasting no more than 14 days.

The ruling limits Buffalo's options. The Sabres must choose between keeping the 19-year-old in Buffalo or returning Grigorenko to a junior team in Quebec.

"We determined that the assignment would not have been consistent with our obligations under the league's agreement with the (Central Hockey League)," NHL spokesman Frank Brown said.

Grigorenko had difficulty earning playing time in Buffalo. Before the demotion, he was sidelined for two straight games and five of seven.