McDonald took over first-team affairs at the weekend after tests revealed Allardyce - who has complained of chest pains in recent weeks - requires an angioplasty, a procedure which involves widening a narrowed coronary artery.
The players only discovered the news last Friday that Allardyce would miss the next three games, but after responding with a 2-0 victory at local rivals Bolton two days later, Roberts believes the squad has shown their professionalism.
"I think the biggest compliment we could pay was that it didn't affect our preparations too much - the other staff stepped in and it was seamless," Roberts said.
"We had a meeting on Friday and spoke about it and we knew what was going to happen on Saturday in terms of our game-plan and the fact that he (Allardyce) wasn't going to be there.
"It's always difficult when the manager is not there, but we have got some personalities in the dressing room. We knew how important it was and we pushed ourselves on.
"There were a few things (to think about) going into the game - obviously it was a local derby and also we hadn't had an away result for a while, so it was big for us.
"We were very happy with the result and I think we played quite well."
The victory brought to an end a sequence of 10 successive away defeats in the league for Blackburn and Roberts thinks the team is now ready to put the statistic well and truly behind them.
"It was a hard stat to take in really," Roberts said.
"But we have got that off our backs now, we go to Craven Cottage and we are hopeful to get some points there as well.
"We know they (Fulham) are a good side - I think they are underrated.
"They are a side that are performing well at home, they are strong and they have some good players so we know it is going to be tough.
"But we have to believe in ourselves at the moment. We are showing decent form, so we need to take that into the game."