Toney was sold to Al Ahli before the international break by the Bees.
Frank, ahead of facing Manchester City on Saturday, said: "Ivan has been fantastic for us over the last four seasons. One of the most important players and his character off the pitch, in terms of driving the team and bringing good energy everyday. He will be missed, no doubt about that, as a player and a person. I will definitely miss him and we wish him all the best for the future."
On his squad without Toney, Frank added: "We got good practice last year. We had, six-seven months, two thirds of the season without him. We also started this season without him. The players have already stepped up and they're growing, like trees.
"In the dressing room, the players need to take responsibility and be leaders. Ivan was a fantastic player on and off the pitch. In general I'm not worried, we have fantastic players, others will step up."