Leko, who made his Baggies debut aged 16, made his Championship debut in the Robins' 1-0 away win at Reading last Saturday.
The 18-year-old hitman came on as a second-half substitute at the Madejski Stadium and did enough to persuade manager Lee Johnson to start him against Wolverhampton Wanderers on Tuesday.
The winger played just under 70 minutes of the match - a thrilling 3-3 draw which dropped Wolves out of the top three.
“He just brings something different to the team,” McAllister said in midweek.
“He’s a real threat in the final third and he causes their defenders real problems - they couldn’t deal with him in certain situations, especially in the box.
“He’s strong, he’s got great feet. Chopping one way and going the other.
“We just felt that he could have got shots off more, and tested their keeper. But it was a great, great debut for him.”