After spending the summer building a side which could compete for promotion, with midfield playmaker McDonald a key component, Weir saw League One rivals Wolverhampton Wanderers trigger the get-out clause.
Discussions dragged on for several days between McDonald and Wolves, with the Blades left in limbo before a deal was struck.
Having netted in the opening-day win against Notts County, McDonald was gone leaving a gaping hole in United’s offensive plans.
The recent arrival of Jose Baxter, from Oldham, looks likely to remedy that but Weir admits McDonald’s exit did cause United problems.
“Looking back, it was just awkward regarding the timing,” said Weir, ahead of today’s reunion with McDonald and a trip to face Wolves at Molineux.
“We were always aware of the clause, knew it could be activated during the window. It was a ticking time bomb, in that respect.
“Obviously , when it did happen, it was quite protracted and it left us a little bit bare in that area but we have replaced him now.
“Jose Baxter will be a great player for the club moving forward, it will just take a little time for him to settle in. Already we have seen what he can bring.
“I have seen Wolves play a couple of times recently, and Kevin is doing really well. He’s a talented footballer.
“We enjoyed having him here and he did a lot of good things, even in the brief time I was here. He chose to move on which was his choice, he had a clause in his contract.
“But the timing for us was not great, it took us a little while to replace him.
“But we wish him well, he’s a good lad, a good player and obviously made the choice to leave this football club.”
A Premier League club just two seasons ago, Wolves are favourites to win promotion back to the Championship at the first time of asking.
“Wolves have got some really talented forward players, wide players, and the level they have got within the group is very, very high,” said Weir.
“I am sure they will be expecting to be one of the promoted teams this season. They came down from the Championship and the level of players and the finances that comes from their relatively recent time in the Premier League, their expectations will be really high, and so they should be.
“It’s a tough place to go, against good players, and in an environment that will be a test for us and the lads. But it’s one we should be looking forward to.
“It’s another opportunity to see where we are as a team, how we can develop as a team, if we can stand up to the test that we will definitely have there.
“They have talented players right through the team.
“It’s important we stand up to that and give a good show of ourselves.”
While the Blades have struggled for goals – just five in eight league games – the recent arrival of veteran striker Marlon King and Leeds United winger Ryan Hall does give them experience in forward areas.
United defender Neill Collins believes that is key if the Blades are to start climbing the table.
“Hopefully, Marlon will just bring what he has done all his career, which is goals. His hold-up play is excellent, his wherewithal, you just can’t substitute the games and goals which he has got,” said Collins.
“Then Ryan (Hall), he looks like he can unlock a defence, put a good ball in and score a goal.
“We have lacked that, there’s no question about that. Ryan has a pedigree for scoring, Marlon King, Jose Baxter, so things are improving. Players that have done it in the past will do it in the future.
“We have improved the squad with those signings and the manager is doing a lot of good work behind the scenes.”