"I said last time (in March) that he was a bit short of matches ― he's had some matches now but we've got players that have done really, really well for us in Adam Lallana and Dele Alli".
The Red Devils were well off the pace in the Premier League, finishing 24 points behind champions Chelsea, only to redeem themselves with victory over Ajax in the Europa League final earlier this week.
"Obviously I'm disappointed, because you play finals to win them", Bosz told a news conference at the Friends Arena.
Every top player wants to play in the world's greatest club competition, so with United now qualified, many will be drawn to Old Trafford this summer.
Manchester United will be playing Champions League football next season after beating Ajax 2-0 in the final of the Europa League on Wednesday. "We've done it, and we're proud", said Pogba. It's right to say we want to qualify for the Champions League, but you know, you want to go try and win that trophy. Obviously I am happy to be part of that [Europa League win] and in some way help the club win trophies, and that is the way it's been over the last 18 months and a year ago.
This season, Rooney has made 39 appearances for United, but had scored just eight goals - the lowest ever of his Manchester United career and the same amount as in his breakthrough campaign at Everton back in 2002/3.
"I think there's a big difference in the physical way they (United) play, but its not a matter of strength - it's how you use your body, which we have to learn", Bosz said.