Chelsea under Mourinho's last spell had no chance for the CL or to be fair had as much chance as any of the last 16 teams. That was 2+ years of his spell and hundreds of million pounds spend on a team that actually won the CL 2012.
When will United challenge for CL? Realistically not before 2018/2019 season, maybe even 2019/2020 if Mourinho lasts this long. They will need at least 6 major signings, provided Martial and Depay stay and reach their touted potential. Mourinho has no patience for this game and his boards seems to have less patience for it either these days. Anyway you look at it, provided we can offer similar terms, Pogba will be mad to leave!