Sunday's 3-0 Premier League defeat at Everton dropped Rovers into the bottom three and boss Sam Allardyce will make wholesale changes for Tuesday's Carling Cup third round tie at Nottingham Forest.
McDonald has told those selected to face the Championship side, most likely to be those who have not had much pitch time so far this season, they are in a position to stake their claim for a regular first XI spot.
Aston Villa visit Ewood Park on Saturday and McDonald said Allardyce's decisions for that game could be influenced by a performance against Forest.
"The manager made some changes for the first [cup] game against Gillingham and with the squad we have and with everyone fit it is
a good time to try to give everyone else a game," he said.
"With the team not doing too well in the Premier League at the moment it gives everyone a chance to stake a claim.
"I'm sure it will give everyone the chance to impress the manager and give him a problem for Saturday.
"If they play well they have one hell of a chance to face Aston Villa."
Despite the anticipated numerous changes McDonald said it was still important to go out and win the game, especially after the disappointment of defeat at Goodison Park.
"The manner of the defeat was most unlike us, conceding goals from set-plays, and so we are smarting," he added.
"We reviewed the video to see what we did well - which wasn't that much - but we have to make sure we don't do it again.
"We have to remain positive. We have to leave that game behind, look forward and do the good things.
"We are always looking to improve and see where we have done things wrong and put them right and make sure the next time we play we don't repeat them.
"Villa is the most important game because that is our bread and butter but we have to get back on track starting against Forest tomorrow.
"All the players want to play in these games but it is the manager's judgement on who he is going to change and who he is going to play.
"He normally gets that right on numerous occasions."
McDonald said they would not be underestimating the Championship side, who have also won just once in the league this season.
"They have got some good players going forward and some goalscorers so we have to be ready for them," he said.
"We will be well prepared but we will have to play to our maximum if we are to get through."
Blackburn 6/5, Nottm. Forest 15/8, Draw 12/5