Wellens is a former United pro.
He told the Manchester Evening News: “More than anything, I'd say you need to get to a stage where Under-23s football is OK and that's all it is.
“Don't stay at that level for too long. The standard is fine but it's not what is required in terms of making a professional career.
“Look at someone like Danny Drinkwater.
“He struggled in his first and second loan spell but took off at Barnsley and his career went from there.
“When you go out on loan you can gauge whether you can come back and make it at United, which is what the likes of Tuanzebe should be aiming for."