Earlier this week Boro agreed a deal to sign goalkeeper Joe Lumley when his contract at QPR expires in June, a shrewd signing given Neil Warnock needed to strengthen his options between the sticks.
Boro are also looking to sign three new strikers this summer and were interested in frontman James Collins, whose contract at Luton was set to end.
Yet the 30-year-old has instead agreed to join Championship rivals Cardiff City, meaning Boro have been forced to assess other targets.
Warnock said at the end of last season he wants to sign around eight or nine new players ahead of the 2021/22 campaign.
Here are some of the names who have been linked with Boro over the last few weeks and their situations as it stands.
2. Joe Lumley - Signed
The 26-year-old goalkeeper became Boro's first signing of the summer this week and has agreed to join the Teessiders when his contract at QPR expires in June. Photo: Getty Images
3. James Collins - Signed for Cardiff
Warnock is hopeful he can sign three strikers this summer and were interested in the 30-year-old frontman. Collins was out of contract at Luton but instead opted to join Championship rivals Cardiff. Photo: Naomi Baker
4. Michael Smith - Rotherham
Following Rotherham's relegation to League One, Smith has emerged on Boro's radar. The 29-year-old wasn't Warnock's first-choice striker option but may have moved up the list after Boro missed out on Collins. Photo: George Wood