Chelsea cannot necessarily better their domestic form from 2014/15, but an improvement in Europe is most certainly on the agenda for Mourinho and his staff.
The London club are already well-stocked in midfield but, with John Obi Mikel looking set to leave Stamford Bridge, as per The National, Mourinho is on the lookout for a replacement and a player of Witsel's ability will benefit Chelsea's squad massively.
The 26-year-old is also allegedly wanted by Juventus, as per ESPN, as he looks set to leave the Russian Premier League. He would have a better chance of competing with Nemanja Matic for the holding role at Stamford Bridge than Mikel, really strengthening Chelsea's options.
It is also worth taking into consideration the fact that Mourinho is represented by the same agent as Witsel, Jorge Mendes, who has a reputation for getting his clients together in several destinations.
Chelsea's approach to the transfer market
Chelsea were by far the best team in the Premier League last season. None of the other clubs came close, which explains why they went on to have a successful season domestically. They have balance, class and strength in numerous departments.
However, Mourinho's men were lucky on the injury front so their approach to the transfer market should be about bettering what they already have and push for more competition within the squad.
Currently speaking, it doesn't look like any other club is ready to take the Premier League crown from the Blues.