As Scrooble said, your example is indeed a comma splice. Independent clauses can't be seperated by a comma alone. In no particular order, here are some options on how to fix it:
A. Semicolon
The 26-year-old Frenchman’s strike rate had caught the attention of Mourinho; his 129 goals in 275 games for Lyon made him an interesting prospect as a successor to Zlatan Ibrahimovic at Old Trafford.
B. Full Stop
The 26-year-old Frenchman’s strike rate had caught the attention of Mourinho. His 129 goals in 275 games for Lyon made him an interesting prospect as a successor to Zlatan Ibrahimovic at Old Trafford.
C. Conjunction
The 26-year-old Frenchman’s strike rate had caught the attention of Mourinho, and his 129 goals in 275 games for Lyon made him an interesting prospect as a successor to Zlatan Ibrahimovic at Old Trafford.
Other notes:
According to New Oxford American Dictionary, "strike rate" is not hyphenated.
According to some style manuals, particularly if you're writing ficiton, "26" would be written out: The twenty-six-year-old Frenchman's ...