Aberdeen "should not" be losing out to English lower league clubs for players, says manager Derek McInnes.
The Scottish Premiership side have sold striker Adam Rooney to National League side Salford City despite being keen to hold on to him.
The Irishman, 30, will earn a reported £4,000 a week in England's fifth tier.
"We have lost out on players to League One, which Aberdeen shouldn't be," McInnes said. "It is a challenge for us to retain our players."
McInnes acknowledged that there would be "surprise" at the fact a club who have only just been promoted to the National League could lure a player from the Scottish Premiership runners-up.
However, he conceded that the money on offer from Salford, who were taken over by a group of former Manchester United players in 2014, was hard to compete with.
"I think Adam has tried to allude to why he went there," McInnes said. "I think what is clear is that Salford are a very ambitious club and are in a hurry to get through the leagues.
"The first conversation I had with Salford was on the very first day of pre-season and they were given no encouragement.
"But it came to the point where we were running the risk of losing the Salford deal for him and I had given him my word that we would let that [the deal] happen."
McInnes will now use the money generated by the Rooney deal to bolster his own squad and says the fee received means he "maybe has the opportunity to bring in one or two more".
With Nicky Maynard being released, Aberdeen were already looking to strengthen in attack, as well as on the right of defence.
"I was looking to bring in another forward anyway and that becomes even more pressing - the timing isn't ideal," said McInnes, whose side start their campaign at home to Burnley on Thursday in a Europa League second qualifying round tie.
Meanwhile, McInnes revealed that he could be without Mark Reynolds for "some time" after the Scotland defender was sent for a scan on a knee injury.