The England Under-19 international and Stoke loanee has made goal-coasting errors in each of the Bucks’ last two league games.
He is cup tied for today’s FA Trophy quarter-final at Solihull Moors having played for Hednesford Town in the competition earlier this season, but has been their No.1 shot-stopper in National League North.
And as Telford try to preserve their play-off place, Cowan said on the 19-year-old: “We didn’t get away with it last Saturday (at Stockport County, losing 3-2) when he made a mistake, and he’s made another mistake (at Ashton United on Tuesday, winning 4-3).
“But he’s a young lad full of confidence and it won’t deter him in the slightest.
“But we do have to get the message to him that he needs to be at the best of his abilities.
“He needs to be right and he needs to be making sure he’s cutting those out.
“Hopefully, that will be the last we see of that now.”
Andy Wycherley was due to be between the sticks instead today at Damson Park.
“There’s no doubt Joe is going to go on and be a very top goalkeeper,” added Cowan.
“He’s going to have a career in the Football League. At what level of the league that will be, I’m not sure.
“But he’s going to be a top goalkeeper. You’re not the England U19 goalkeeper for no reason.
“Stoke have got high hopes for him and when you see him day-to-day, you see what there is there.
“He’ll be thinking Tuesday is something for him to learn from, but I’m saying that needs to be cut out now.
“We’re getting to the business part of the season, so he’s got to make sure he’s on his mettle.”