It was the Croatian forward's first strike for the club since joining from Hajduk Split in the summer and came just minutes after he had set up a goal for Benni McCarthy.
McDonald admitted that it had taken time for Kalinic to get to grips with the demands of the English game, but said that his performance against Posh gave a clear indication that he was finding his feet.
"It shows that he has got vision," said McDonald.
"He has created a couple of good chances as well and to get that first goal will take the pressure off him of course, but that's against Peterborough.
"It takes time for a foreigner to come into our English game and adjust to the pace, to the style of play and the physical and mental demands.
"He has been patient and we have seen glimpses of what he can do certainly. He'll only get stronger I'm sure."
McCarthy had originally been in line to take Kalinic's penalty - Blackburn's second of the night after Reid's first-half conversion - but McDonald revealed that the South African had decided to reward his 21-year-old strike partner for the earlier assist by allowing him to take it instead.
"Steven Reid was always going to take the first one, and then I thought it was a really nice touch because Benni was supposed to take the second one, but Nikola has knocked the ball through for him and he's stuck it away nicely and Benni wants Nikola to do well." McDonald added.
"So he gave him the second penalty and he stuck it away.
"I'm sure if he had missed it there could have been a huge problem, but he has stuck it in the back of the net and showed that he has got the confidence to do it.
"If it is only from 12 yards or from open play, we want him to score goals and that is his first which is great."
McDonald was also keen to praise the display of Reid, who scored his first goal since returning to first-team action from a long-term injury.
"When you've had an injury for such a long time, you always get a few more niggling injuries and you get a little bit frustrated, but he's the model professional," said McDonald.
"He's worked very hard to get back and he was always going to play around about 75 minutes today, but it's an extra bonus that he has contributed a goal.
"You can see what his strengths are - he is a good passer which is great, and hopefully he will be able to contribute over the next few months to get us up the league this season."
Reid celebrated his goal with the club's medical staff and McDonald paid tribute to their work in getting the Irishman back to full fitness.
"They worked tirelessly to get him fit, in the gym and pool," McDonald said.
"After everybody has gone home, they have worked very hard and that was just a little thankyou to them.
"The goal caps off a good performance from him and it's nice for him to show that appreciation for the people who work behind the scenes."
Posh boss Darren Ferguson - who saw his side equalise twice at 1-0 and 2-1 down - was unconvinced by the referee's decision to send off goalkeeper Joe Lewis for the foul on Morten Gamst Pedersen which resulted in Reid's penalty.
"It's disappointing for the fans and it's disappointing for everyone," said Ferguson.
"It's not as if we came here and got annihilated. Obviously as the game went on, they kept the ball and I started to become more concerned about us keeping some energy for Saturday.
"But we gave them a game up to the sending off, and that was the turning point.
"I would have to see it (the sending off) again - I have only seen it once live.
"He (Pedersen) made his decision and Joe tried to get out of the way of him, you can see in his body shape.
"But he has just nicked it around him, kicked it out of play and the ref has sent him off and given a penalty."